Merge pull request #501 from kagemomiji/issue254-add-coverart-doc

#254 add document for cover art

docs/media/coverart.md

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+# Cover Art/ Artist Image
+
+Airsonic-Advanced supports cover art and artist image for your music.
+
+## Configuration
+
+Settings for importing cover art and artist images can be found in the WebUI under `Settings` > `General`.
+
+| item | Description | Default Value |
+| --- | --- | --- |
+| Cover art files | The image file names and extensions to be used as cover art, separated by spaces.| cover.jpg cover.png cover.gif folder.jpg jpg jpeg gif png |
+| Cover art source | The source for obtaining cover art.| Prefer external file over embedded image |
+| Cover art quality | The quality of the cover art.| 90 |
+| Cover art concurrency | Specify the number of cover art thumbnails that can be generated simultaneously (if needed). Higher number means more thumbs can be generated simultaneously, but requires more CPU threads/cores. This setting requires a restart before it takes effect.| 4 |
+
+
+## How it works
+
+Airsonic Advanced handles cover art and artist images under the assumption that files are saved in an artist/album/song folder structure as follows:
+
+### Cover Art for Music Files
+
+Airsonic Advanced handles cover art as follows:
+
+1. Retrieve cover art according to the method specified in Cover art source:
+  - Prefer embedded image over external file: Prioritize the embedded cover art in the file.
+  - Prefer external file over embedded image: Prioritize the image in the same directory as the file.
+  - Use only embedded image: Use only the embedded cover art in the file.
+  - Use only external file: Use only the image in the same directory as the file.
+2. If cover art is not found using the above methods, the default cover art is used.
+
+### Cover Art for Album
+
+The album cover art uses the cover art from one of the music files directly under the album.
+
+### Artist Images
+
+Artist images use images with the `Cover Art files` in the artist directory. If none are found, a default artist image is generated.
+If you set the `Cover art source` to `Use only embedded image`, the artist image will not use files in the artist directory.