ED saves screenshots as bmp, I don't think you can change that.
You can't upload images (in any format) direct to the forums, it was disabled ages ago (maybe one day they'll actually delete the dialog that makes you think you can).
You'll need to upload them to a dedicated image hosting sight (imgur, flickr, etc...) and us the 'From URL' tab on the Insert Image control (or just wrap it in BBCode IMG tags manually).