Okay, I used this method, from Irish in the Military siggy thread:
"This is mine:
Upload the pic to a public place (photobucket, TK "my photos", whatever), paste the URL into the spot above, and paste that whole dealie into your siggy, under "My knot" > "My bio" > "Update my siggy" HTH!"
I used photobucket, pasted in the URL where it says "your URL here" but it's still not visible. Does another photoshare site work better, or am I just as computer illiterate as I think?