The new Gimp is stupid like that. You can't save-as and make it a jpg or png. You have to export it. Which is stupid, because it is exactly the same thing. And I have been a Gimp user for a couple of years now, and I still unconsciously press save as instead of export. But that is off-topic.
Also your title is asking how to post a pic, yet in the description you are asking how to export. Remember to think before you post, or you'll start confusing people.
And also if you are having problems uploading the photo to the forums. Try
this.