How to reproduce.
- Copy any non-image binary file to 'test.bmp' to imitate broken image file
- Run gnumeric
- "Insert->Image"
- Choose your 'test.bmp' file
- Click anywhere on sheet to insert it
- Click somewhere also on the sheet to deselect an 'image'

 Now you have a small object that is quite hard to remove (much easier to remove cell or row/column where it was placed).

Patch for quick fix is attached.

It would be better to improve gnm_soi_new_view or soi_get_pixbuf,
if pixbuf == NULL it have to show some warning dialog and cancel image insertion instead of copying 'unknown_image' placeholder.