Thumbnailers for Krita (.kra), Gimp (.xcf), OpenRaster (.ora) and Adobe Photoshop (.psd) files
19.04.2015
If you want thumbnails in nautilus (the file manager, that is part of GNOME and used in Ubuntu by default, nowadays it's just called "Files"), nemo (cinnamon I think) or pcmanfs (xfce) for one of the formats mentioned in the title, it's actually quite simple to add them.
Nautilus and some other file managers run scripts and executables to create their thumbnails, and the associations are controlled through files located at /usr/share/thumbnailers/ in your usual Linux distribution.

Simply create the files below inside said directory, clean the cache (rm -rf ~/.cache/thumbnails) and restart your file manager. For nautilus for example, you can run "nautilus -q" (-q for "quit") in a terminal to terminate all running instances of it.
File: /usr/share/thumbnailers/kra.thumbnailer
[Thumbnailer Entry]
TryExec=unzip
Exec=sh -c "unzip -p %i preview.png > %o"
MimeType=application/x-krita;
File: /usr/share/thumbnailers/ora.thumbnailer
[Thumbnailer Entry]
TryExec=unzip
Exec=sh -c "unzip -p %i Thumbnails/thumbnail.png > %o"
MimeType=image/openraster;
File: /usr/share/thumbnailers/psd.thumbnailer
[Thumbnailer Entry]
TryExec=convert
Exec=convert psd:%i[0] -scale 512x%s png:%o
MimeType=image/vnd.adobe.photoshop;
File: /usr/share/thumbnailers/xcf.thumbnailer
[Thumbnailer Entry]
TryExec=xcf2png
Exec=sh -c "xcf2png %i -o - | convert -scale 512x%s - png:%o"
MimeType=image/x-xcf;