Hmm I'm sure it's possible in photoshop or what have you, but if you don't have that/have no idea how to use it (like me XD) then you're better off just cropping an image in MSPaint. If it's a long pic and you don't wanna miss parts of it then it's better off in your siggy
When you upload to Imageshack or Photobucket you can choose to have your image resized. Though the only option that is small enough is 100x75. You can easily resize using Gimp though, which is free.