Also, I made an attempt at a instagram inspired client for Mastodon a while back. It's unfinished, but live on http://zatnosk.dk/playground/mastogram/
The code is available on https://github.com/zatnosk/mastogram/
The main thing I'd want you to draw inspiration from is how I present multiple pictures in one post. I think my algorithm can be optimised to scale-fit the images better, though.