Miss Penny: Only you can do this, I'm afraid.
You must ensure the photo you want to use is the proper size and saved on your computer in an easy-to-find location.
Then go into your "Edit Profile" mode and you browse for your new picture in the "Upload My Own Pic" field. (You removed the old one from "I Have My Own Pic" field, which is where the forum saves your avatar. You are adding something new, so you have to select from the other field).
Since you've eliminated one photo, you've done that much right.
You then go to the bottom and click on "Change Profile" to save your change.