Step 1: go to Gravatar, and sign up to the site with the same email address you used for your pokemondb account.
Step 2: Upload an image from your computer or copy the link of a picture from a site.
ex. "http://www.gravatar.com/avatar/4e929058861aa6ba1955b550748de063?s=200" gives this image.
Step 3: after your picture uploads, you can choose to crop it, so change it so it looks good. If it's too big, then it won't show up well for your profile.
Step 4: select a rating for your image. For this site, you'd obviously have to pick an appropriate picture, but that one was obvious.
Step 5: After your picture is done, choose that picture as your main picture on gravatar.
Step 6: return to pokemondb, and on your profile, choose "my account." Under the "my avatar" section, one of the options is "show my gravatar." Choose this one and your picture will come up. It'll take a few minutes and won't come up right away though.
Step 7: Enjoy! If you want to use a different picture, just repeat everything above using the new image. (gravatar lets you hold up several images and switch them around.