fixed problem Arlo found where tighter layout mode causes the
	selection boxes of adjacent icons to overlap, by giving a little
	extra space in tighter layout mode.

	made thumbnails update in raster order, instead of reverse order
	like it was doing, by using glist_append instead of glist_prepend;
	also, made it use the real "broken image" icon.

	added image from Susan to use as "thumbnailing failed" image