I am new here and when I signed up I did not have a photo to post

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When I signed up at my lot I did not have a photo available to post for my avatar so I used the one from my lot. Now that I have some photos to choose from, I cannot seem to change my avatar. Could someone please be generous enough to tell me the correct way, from start to finish to change my avatar? Thanks....( am feeling kind of stupid about now LOL)