first time posting a pic on here from my iPhone, but it won't let me browse to pick my screenshot from the attachment pop uphelp?
OP...I'm sorry to burst your bubble but you've been beaten to the punch!
https://forums.macrumors.com/threads/543903/
You can't. You have to do it on your Mac/PC. Either email it to yourself or sync it to iPhoto, and then you can upload it to the thread.