Uploading my picture

Hi folks, I was trying to up load a picture of me on holiday by the River Loire, but although i went to the correct page in "My account" browsed to find "my pic" and hit the upload button it just cleared the browse box and nothing happened. Is there a file size limit? Or could it be my firewall? Or am i just overlooking something basic? I see that not many other people have put their pic online and use their default which suggests this may be a common problem. Any help appreciated. Thank you. Howard