Why images I use in the PictureFrame command drop out after a while?

I’m using the command “PictureFrame” to paste images of marble slabs, so as to create a more realistic view of walls & stone works for our clients.
After a while some images just drop out indicating the file can’t be found?
The file is still in it’s original location and the rhino file hasn’t changed either.

Any clues??

Thanks Piet Boer Sydney Australia