What picture editing software do you have? If its just MS Paint you are pretty limited.
But if you have something like Paint Shop Pro, or Photoshop. Then have a look under the menus for "Resize image" enter the picture size you want. So if its for here then enter a pixel size of 100x100....or if one size is larger than the other (eg. the picture is taller than it is wide) then set the longest side to 100 and tick the tick box to "keep aspect ratio" that way the image wont look squashed.
Then save it, if needbe drop the quality slightly so that the file size drops below 58Kb.
Then upload as per normal.
Hope that helps buddy.