If you want to change or upload a picture for your profile (ie: the one that appears against all your blog posts) then you do that from the right hand panel options on your home page when you log in.
This is different from the MY PICTURES page - which is more a sort of gallery of images for your blog posts or for the site's picture albums.
I'll add a link to make this clear in the MY PICTURES page at some point - I've spotted a couple of folk understandably missing this distinction.