ok, first of all, it can't be bigger than 150x450 pixels. to check that it's smaller, you should open your image in paint and go to edit and drag down to select all. in the bottom right hand corner there should be some numbers (ex:64x135) if theyre smaller than 150x450 then it's ok. if they're higher, then make the image smaller by clicking the corner and dragging inward. then go to your profile here and click on the tab edit options(or edit profile, i forgot which one). right under the signiture box there's a link thingy that says upload signiture image. click on that and you get to a page that has a browse bar. click on the browse button and find your image from your own files. usually they're under "my pictures".
if that doesn't work, go talk to the people in the signiture forum because like i said, they're brilliant 😄