I found a triplane at http://lykas13.deviantart.com/art/You-okay-Rainbow-Dash-356776547?offset=0#comments will it work? also http://gonein10seconds.deviantart.com/art/RAINBOW-RAPTOR-DASH-203917931 for the jet. Although i dont think i can ask permission for you.

I've finally taken the step of making a deviantArt account, so a link is fine there. I've removed some upcoming things from the list above; I'll add other stuff later.

I am currently trying to rally some artist friends of mine to your cause.
You know, you should also ask for custom Leader pic ponfications as well. Who knows? You may just get them!

Thank you! I'll look forward to seeing what comes of that.
Leader pic ponification is not a bad idea per se, but those pictures have to fit more requirements to work well. They must look good at 1600x900, must work with the various menu overlays and ideally should look like the particular leader at a glance. (I've occasionally had the thought of Aztec led by Ahuizotl, but for the most part they'd have to be the original leader ponified.) I would be surprised if more than a couple of suitable pieces existed. They could of course be made specifically, but if people are going to make art for this project specifically then leader screen pics are pretty low-priority in my opinion.

I was assuming that the pieces in question would be ponifications of the leaders themselves (which would just be awesome). What about things like the little leader thumbnails? (Like when choosing a civ.) Would those be possible to change?

Certainly. Those are a bit easier actually.

Do you want it exactly 256x256?
Anyways, cropped right out of "Ponyville Confidential" (Around 1:00).
Thought it would make a good "Sailing": http://i.imgur.com/1ehtrSI.png

Nice, thank you. And no I don't need exactly 256x256, I just need at least that since that's the largest icon size. I can easily shrink down bigger things.