I encountered this problem when lxde-meta was emerged without having set the "desktop" profile.
This means that the 'jpeg' USE flag is not set (along with all other imagine flags).
LXDE can still be installed, but when it pulls in the package "gdk-pixbuf" (responsible for loading images) it is compiled without jpeg support. This causes the black background (image can't be loaded) and also the default image viewer (gpicview) being unable to open jpgs. PCManFM thumbnails are also affected.
It can be fixed by running:
euse -E jpeg jpeg2k
emerge -UDua @world
To add the 'jpeg' & 'jpeg2k' USE flag globally and then re-emerge any packages with USE flags which have changed since compilation.
I also needed to recompile the dependencies of "gdk-pixbuf" by running:
emerge -ae gdk-pixbuf
This shouldn't be required as far as I can tell, but is likely needed due to necessary libraries not being in place if only the packages affected by the USE flag are re-emerged.
Possible USE flags for "gdk-pixbuf" and thier current state can be easily found by running:
equery uses gdk-pixbuf