There are two popular ways to "Make it big"
First. Use an image hosting site, and just tell it to post reference the image (un-check the grab from url box).

Second way:
Attach the pic to your post.
Then, view your post, and click on the pic to view it larger. Grab THAT url and copy it to the clipboard (Android will offer to do this with a long press on the image).
Edit your post, stick another image in there, but un-check the grab from url box and then paste the url in the clipboard into the image url. Save your edit.
It will now show the referenced larger image in the original post