Learning by doing honey.......
There must be a "open file" function (I haven't downloaded the program yet, since I use ACDSee) somewhere. Find it and then try to locate your picture. Open it. Now you need to look for something like " Resize " or "Change" (a look into the help files might do wonders....
).
With this function you change the size to the required size here (I think it's 100x100 bixel, or somethin' link that).
After changing the size, look for "save as..." Save it as .gif onto your desktop.
Then you go back here and try to upload it. If it works, then you're fine, if not, then you have to rezise it again till it fits the rules......