I appended a file which a user uploaded to blendpolis. He claimed that he can't select an image for the current selection. I successfully reproduced it in r50278. If i open the file then i have a mesh with already some faces selected. If i now try to assign a image nothing happens (at least visually). Opening another image from the disk does also not work. I can load it, but nothing happens if i select it. To make it work again i have to change the selection in some way. After that i can assign the images as usual. My only guess would be that the selection is somehow corrupted or not recognized by the image editor.