Welcome! I think you may have to have a minimum number of posts before you can put an avatar picture up. That may be true for posting pics in the content also but I'll tell you what I do.
In Photobucket I choose the "direct" photo URL and copy it (the "image" URL should work also too). Then, here in Arfcom, I click the "photo" button (it's the one fourth from the far end, looks like a pic of a mountain with a sun in the corner). When the prompt pops up where you enter the URL, I delete everything already entered ("http://" if I remember correctly) and then paste my URL and click "OK". That works for me. There are some sights I can just put the URL into the body of the post and it work but I can never remember which sites do that and which don't so now I just use the standard method on everything.
To access the spot to insert an avatar pic, at the very top left of your screen, you click the little person button between the red log in/out button and your screen name. Once that page comes up you click the "main" button that appears directly under the "Information" section. After clicking the "main" button you'll see the section at the bottom of the page labeled "avatar". Directly under the "Avatar" section is the button to change avatar pics. As I said though, you may need to meet a minimum post count before it lets you do anything.
Hope that helps some.