For the season 3 caps of 9-1-1 Lone Star I used *.mkv files and they did fine. I use it to watch (not screencap) a lot of fanvids I've downloaded throughout the years and they have a multitude of video formats.

I uploaded texture sets I made to mediafire.com and as I have 10GB available with a free account, I intend to upload the screencaps there. I just need to whittle them down a bit more so they don't take up too much space.

As for the 7zip - I typically create *.zip files to keep it compatible for a lot of people. I tend to prefer it due to the options it gives me when opening zip files.

I'd be happy to answer more questions if I've run into that before and have an answer for it :)