[Suggestion] Allow for cover* and example images in (at least) PNG, WEBP and SVG formats

[Olf0]

Check first, to which extent this is already the case.

Notes

• For cover* images in SVG format, there is no reason to distinguish between cover and cover_big. The easy workaround is to let cover_big be a symbolic link to cover, but more elegant would be an adaption of the responsible code.
• Actually there appears to be no reason not to support all image formats Qt supports (see [1] and [2]), though practically only JPEG, PNG, WEBP and GIF are relevant.

In either case, the documentation shall be adapted.

P.S.: See also issue #93, which builds on the implementation of this issue (#65).