Manually add thumbnails, posters and fan art to XBMC items

If you don't have an internet connection on your media center, or don't want to use the automatic download of media, here is how you can manually add thumbnails, posters and fan art to XBMC items.

Use JPG or PNG (with transparency) image-format, and size of the picture to anything from 128×128 to 256×256 in pixel size, (the larger the image is the better it will look on a good display but at the same time large thumbnails loads slower).

XBMC will cache all thumbnails to userdata\Thumbnails\. In order for your custom thumbnail to show, you may have to delete the cached thumbnail.

Individual items inside a folder (bust be named .jpg, even if it's .png):

Movies\path\moviename.avi
Movies\path\moviename-poster.(jpg/png)
Movies\path\moviename.jpg

Folders:

Movies\path\foldername\
Movies\path\foldername.jpg

or:

Movies\path\moviename\folder.jpg
Music\path\album\folder.jpg

Music:

Music\path\audiofilename.mp3
Music\path\audiofilename.jpg

And playlists:

Music\path\audioplaylistname.m3u
Music\path\audioplaylistname.jpg
Music\path\cuelistname.cue
Music\path\cuelistname.mp3
Music\path\cuelistname.jpg
Music\path\shoutcastlinkname.pls
Music\path\shoutcastlinkname.jpg
Music\path\audiostreamname.strm
Music\path\audiostreamname.jpg

Relevant information collected from XBMC Wiki