H.W.
J.,
I am typing this for my husband-- I asked him for his male opinion. He says:
" Dude, you were like five in that photo. If you were 15 it would probably be embarrassing. Your mom loves you and wants a photo to remember you by. You should make her a copy of the photo and send it to her as quickly as you can. Your mom wouldn't ask for it if she didn't love you."
J., I rarely ask for my husband's opinion for questions on this site, but did want you to have a male perspective. Do it. When you are 50, like my husband is, you will be SO OVER the underwear issue. Chances are, that 5 year old in the photo barely looks like who you are now.:)
If it's any consolation, my grandfather used to tease me "I have naked pictures of you-- don't forget." He did-- I was two, bare naked on Waikiki Beach where I grew up, and I was on the cover of a travel brochure. LOTS (thousands!) of people saw that brochure. Wish I had my own copy, come to think of it.