Hi Laura,
You can change your profile photo by first signing into Fixya and then clicking your avatar in the top right corner of the page. Select the My Account link in the dropdown that appears.
On the My Account page scroll down to the My Settings section of the page and click the upload button below the avatars displayed.
In the popup that appears select the file from your computer that you would like to use as your photo. After selecting the file you should see the image in a large box.
You can then crop the photo and you should see the thumbnail next to the larger image. Once you have cropped the image as you'd like, just click the blue "Upload photo" button in the popup window.
After the window closes, click the "Save my information" button in the settings section.
Hope that helps, but please let me know if you have any further issues and I will do my best to help troubleshoot.