[Doxygen] Deduplicate placeholder table and document image overrides

- Make custom_card_pictures.md the canonical home of the URL reference-point
  table; loading_card_pictures.md cross-references it through @ref instead of
  maintaining a second copy (unaddressed review comment).
- Switch the remaining @subpage custom_card_pictures to @ref in
  fixing_card_pictures.md so the page keeps its single parent under
  user_reference.
- Document the Image Overrides feature added in #7311/#7312 on the user page,
  loading_card_pictures.md and fixing_card_pictures.md: local override storage,
  the downloadedPics root lookup, exact file-name matching, and the set-folder
  vs flat export naming schemes.

doc/doxygen/extra-pages/user_documentation/card_pictures/custom_card_pictures.md

@@ -1,10 +1,11 @@
 @page custom_card_pictures Custom Card Pictures
 
-There are three ways to make Cockatrice use custom artwork for your cards:
+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 @ref fixing_card_pictures instead.
 
@@ -109,3 +110,26 @@ 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
 @ref 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.

doc/doxygen/extra-pages/user_documentation/troubleshooting/fixing_card_pictures.md

@@ -28,7 +28,7 @@ valid URLs. If you suspect the list has been modified or corrupted, press 'Reset
 defaults.
 
 For information on how to add your own custom URL templates, see the 'How to add a custom URL' link in the same
-settings section, or @subpage custom_card_pictures for a full reference of the URL reference points, the CUSTOM
+settings section, or @ref custom_card_pictures for a full reference of the URL reference points, the CUSTOM
 pictures folder, and custom card databases.
 
 # Check Your Local Picture Folder
@@ -42,6 +42,9 @@ Cockatrice checks the following locations, in order:
 - The custom pictures folder (recursively indexed by file name).
 - `<pictures directory>/<set code>/<card file name>`
 - `<pictures directory>/downloadedPics/<set code>/<card file name>`
+- `<pictures directory>/downloadedPics/<card file name>` (for export naming schemes without a set folder)
+
+A file only matches when its name without the extension equals one of the recognized scheme patterns exactly.
 
 The following import naming schemes are recognized (using both `_` and `-` as separators). The canonical table with
 concrete example file names is on @ref custom_card_pictures:
@@ -58,6 +61,10 @@ If a picture you downloaded or placed manually is wrong, stale, or corrupted, de
 attention to the `downloadedPics` subfolder: this is where the filesystem caching method writes downloaded images, and
 after a provider outage it can permanently contain the wrong printing until you delete it manually.
 
+If a card persistently shows artwork you assigned yourself, you may have an **image override** set for it. Right-click
+the card in the deck editor's printing selector and use 'Image Overrides' → 'Clear Custom Image' to remove it (or
+delete the stored `.png` in `downloadedPics/`). See @ref custom_card_pictures for details.
+
 See @ref loading_card_pictures for details on how local images are loaded.
 
 # Clear Caches