Anything which Qt recognizes as an image. You can see the list by running the command:
Code:
calibre-debug -c "from PyQt5.Qt import QImageReader; print([bytes(x).decode('utf-8') for x in QImageReader.supportedImageFormats()])"
Aside: Linux distro packages may be different than the official calibre releases. My from-source Arch Linux install with system Qt5 only lists [u'bmp', u'cur', u'gif', u'ico', u'jpeg', u'jpg', u'pbm', u'pgm', u'png', u'ppm', u'svg', u'svgz', u'xbm', u'xpm'].