If you haven't figured it out yet, read on. :)
The way I do it it's this:
I go to the meme I created, tap and hold, and click "save photo" or whatever it says.
Then click add meme as you reply, click "upload image" and upload that image from the gallery.
Because it won't allow you to post av empty meme, tap "more options" and set the max font size to 0 and type something into your photo