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Sooo... did you browse to an image on your computer? Was it of an appropriate size? Did it get uploaded to Trains dot com?
I have not updated my avatar recently, so I don't remember all the steps (or pitfalls) so I can't do much more right now... if you could expound on what you did and how the computer responded I could try to replicate it to see what I see on my computer and we can compare notes.
Given that you have another posting about problems with paragraphing in postings, I wonder if you have some other setup in an odd state or if maybe you have some piece of software on your computer that is conflicting with the browser's text inputting.
What computer are you using... PC or MAC or ? ?
What operating system.... OSx, Winders Vista, XP, 7, etc. ?
What browser... Opera, IE-?, Firefox, etc.?
What anti-malware are you running?
I just went through the process and was unable to CHANGE the avatar until I had REMOVED the one in use at the time.
Then I was able to upload a different image and the system took it.
You might try the Remove Avitar and close the dialog box and then try it again.