I’ve seen a few examples on file upload from WP7 – there a good answer here to upload a file. But in my case I’m sending the file to a 3rd party who expects from form fields with the POST. I haven’t seen any examples of this yet.
There’s a codeproject article here, that explains how to do it in non-silverlight friendly code.
I could port over the codeproject article – or does this already exist somewhere?
Search “mutlipart form” to find resources on the subject. I think that’s what you’re searching for: http://www.stackoverflow.com/questions/5622900/wp7-post-form-with-an-image