Go to imageshack.us > press browse > this will open up a window > select the file you want > click ok > check to remove the size banner > upload > scroll down on the new page > you'll see a little +advanced coding+ or something > click it > copy the direct url > go to edit your signature > at the top of the signature there should be 8 boxes, started with b, i, u > the fifth box is a little picture > click the picture > a little box pops up > paste the code your copied from imageshack > save your signature.
Be sure it's not above 60kb in size (if it is, I'll tell you

) and 500 (w) by 200 (h) .