There are four ways to make Cockatrice use custom artwork for your cards:

Placing image files in the CUSTOM pictures folder.
Providing a custom card database that points each printing at a picture URL via the picurl property.
Writing your own download URL templates.
Setting an image override for a single card from inside the deck editor.

Each of these is described below. If pictures are missing or wrong, see Fixing Card Pictures instead.

Custom Pictures Folder (CUSTOM)

Any image file placed in the CUSTOM folder is used as the card picture, and no download is attempted for cards that match a file there.

The folder is <pictures directory>/CUSTOM/. The pictures directory is configured on the 'General' settings tab, under 'Directories' → 'Pictures directory'.
Any image format Qt can decode is accepted (PNG, JPG/JPEG, WebP, GIF, BMP, ...); the file extension is not filtered, so even an extension-less file is picked up if the decoder recognizes its content.
Files are indexed by their name, so you can organize them into subfolders freely.
New or changed files are picked up automatically within a few seconds — no client restart is required.

The file name must match the card using one of the naming schemes below. Both _ and - are accepted as separators, and the file extension is ignored when matching:

Scheme	Example file name
Card Name	Example Card.png
Card Name + Set	Example Card_DDL.png
Card Name + Set + Collector	Example Card_DDL_43.png
Set + Collector + Card Name	DDL_43_Example Card.png
Card Name + Provider ID	Example Card_0b23cdc8-d413-4fb1-8470-474221b10fe2.png

The name used for matching is the corrected card name. Correction removes the split-card separator // and the characters reserved in Windows file names (* < > : " \ ? and control characters), and turns / into a space, so the "Example // Card" card is matched by a file named ExampleCard.png, not Example // Card.png. Most other punctuation (commas, apostrophes, !, ...) is left untouched.

Attention: A file in the CUSTOM folder always wins over downloaded pictures, even if it is the wrong image. Delete the file if you want to see the downloaded artwork again.

The naming conventions are the same as those recognized in the set-named subfolders and in downloadedPics, and are documented for developers in Loading Card Pictures.

Custom Card Database (picurl)

If you maintain your own card database (see the Custom Cards & Sets wiki), each printing's <set> tag can carry a picurl attribute containing a full URL for that printing's picture:

Cockatrice tries this URL before the configured download URL templates, so it is the most direct way to provide custom artwork for a specific printing.

The URL should start with http:// or https://; the scheme is not validated, so make sure it is absolute or the download may silently fail.
When you change a picurl for a card whose picture was already downloaded and cached, delete the stored images (Storage tab → 'Delete Saved Images' / 'Delete Cached Images') so Cockatrice fetches the new URL.

Custom Download URL Templates

The built-in download URLs are templates: Cockatrice replaces reference points in the URL with information about the card and its printing. You can write your own templates in 'Cockatrice → Settings' (Ctrl + Shift + P by default), on the 'Deck Editor' tab, in the 'URL Download Priority' section.

The following reference points are available:

Reference point	Description	Example
!name!	Card name	Example Card
!name_lower!	Card name, lower case	example card
!corrected_name!	Corrected card name	ExampleCard (instead of "Example // Card")
!corrected_name_lower!	Corrected card name, lower case	examplecard
!sflang!	Scryfall language code for the current client language; defaults to English when the language has no localized images	en, zhs
!setcode! / !setcode_lower!	Set code	EXM / exm
!setname! / !setname_lower!	Full set name	Exemplary Set / exemplary set
!set:<property>!	A property of this printing, e.g. muid (Gatherer multiverse ID), uuid (Scryfall UUID), num (collector number), rarity	373549
!prop:<property>!	A property of the card, e.g. side (front/back), colors, cmc, coloridentity, type, pt, and the format legality statuses	front

The !set:...! and !prop:...! reference points support two modifiers:

_fill_with_<text> pads the value with the given text, right-aligned, e.g. !set:num_fill_with_000! turns collector number 1 into 001. If the value is longer than the fill text, the template is skipped.
_substr_<start>_<length> extracts a substring, e.g. !set:num_substr_2_2! takes two characters starting at the third. If the substring would extend past the end of the value, the template is skipped.

Substituted values are URL-encoded. A template that asks for a property the card or printing does not have is skipped, and the next template in the list is tried instead.

Attention: Custom URLs should start with http:// or https://. As with picurl, the scheme is not validated before the URL is handed to QNetworkAccessManager, so use an absolute URL or the download may silently fail.

Some working examples:

https://cards.scryfall.io/large/!prop:side!/!set:uuid_substr_0_1!/!set:uuid_substr_1_1!/!set:uuid!.jpg
https://api.scryfall.com/cards/!set:uuid!?format=image&face=!prop:side!
https://api.scryfall.com/cards/multiverse/!set:muid!?format=image
https://gatherer.wizards.com/Handlers/Image.ashx?multiverseid=!set:muid!&type=card
https://gatherer.wizards.com/Handlers/Image.ashx?name=!name!&type=card

See the Custom Picture Download URLs wiki for more examples and ideas.

Attention: Keep in mind that templates using !name! or !set:muid! resolve by name or multiverse ID, not by the exact printing. Only the Scryfall !set:uuid! templates always return the exact printing requested. See Fixing Card Pictures for more on this.

Image Overrides

The quickest way to give one card custom art is an image override: right-click the card in the deck editor's printing selector and open the Image Overrides submenu of the context menu.

Load Custom Image... — choose a picture file (the dialog suggests PNG, JPG/JPEG and WebP); it becomes that card's artwork immediately.
One entry per alternate printing of the card, labeled <set> <collector number> (hovering an entry previews that printing's artwork). Selecting one makes the card use that exact printing's picture, so e.g. a basic land can be shown with any of its artworks.
Clear Custom Image — removes the stored override and returns the card to normal resolution. It is only available while the card has a stored override.

Overrides are stored as .png files in <pictures directory>/downloadedPics/, under the "Naming scheme" configured on the Storage settings page, and are matched the same way as downloaded images. Because local files are checked before any URL is requested, an override always wins over downloaded artwork and picurl for that card. The override exists only on the machine it was created on - it is not part of the deck file - so a card with a stored override shows normally on another computer.

Attention: If you also keep a matching file in the CUSTOM folder, that file is matched before the override. When you change an override, use Clear Custom Image so the stored .png is replaced; manually deleting the file in downloadedPics/ has the same effect.