With modern digital cameras generating high resolution images, it seems
very easy for gthumb to fill ~/.cache/gthumb

For example, when working with 50MB raw images from a Nikon D800, I
observed gthumb creating 100MB pnm image files in ~/.cache/gthumb

When ~/ becomes full, gthumb seems unable to create more cache images,
and simply doesn't display any more images.  The UI remains responsive,
it is possible to scroll the thumbnails, but the image displayed in the
main viewing panel is the last one to be accessed before the disk filled
up.  No error is given (either in the GUI or the shell where gthumb was
started) to suggest an error creating cache files

A user not monitoring their free space would find this very frustrating
and would have no idea why gthumb is unresponsive.