When you press Reply and the box is open for posting, look down below next to Attach:
press Browse... and your selection of photos should be available. Perhaps you already tried that lol
But I usually go to Photobucket and copy/paste mine here in the reply box... the pics
Yes, as Tana said above, touch 'choose file' then 'choose existing' and your photos will come up. From there you can choose. If you want more than one image, touch 'more attachments' and then just repeat the process: