Blunderer it took me ages to get my head round it, try something like this: Load the Image you want to use into something like MGI PhotoSuite or Paint Shop and resize it to 100 pixels wide by 120 pixels high. Or if your feeling masochistic you can always load it into M$ Paint and mess about with Stretch/skew then checking the attributes, when its near the allowed size do the slight adjustments with the size boxes in attributes. Does any of that tripe make sense?
If you get really naffed off email it to me and I?ll send it back the right size.
Good luck and I hoped that helped in some way.