When I go to the video options in ZSNES, there is no 640x480x8B VESA2 mode, which DC mentioned. If it helps, I'm using the windows version of ZSNES (though it shouldn't make any difference).
Problem ripping tiles...
Personally, I use snes9x. And if it becomes too much of a hassle, press the "Print Screen" button to make a copy of everything shown on the screen at that point, bring it to paint, extract your files (I know Don M's RM2K place has dividers so you can make sure it exact.) Bring to tileedit, paste, and, well, its a hassle, but you did it.