Oh I just need to practice with it a little and maybe watch a couple of youtube tutorials - I have an unofficial aunt whose amazing with Photoshop so I may ask her advice.
Good idea. You'll get the hang of it quickly enough. All the tools are labelled and placed where you'd expect, I had no prior experience with anything more than Microsoft Paint and now I can make wallpapers and do manually done depth of field on photos. It shouldn't take too much time to learn stuff like Select by Colour that you might need for GC3 images.
Do you know what resolution the main pics and leader thumbnails need to be? I know they need to be .PNG files - is that right?
PNG files, yep. The aspect ratio is what's important, but I can't remember them so I'd say to just do 1920x1080 for foregrounds and backgrounds, about 128x128 for logos and 532x532 for leader portraits.
[stuff about shipsets and factions]
Ships can only be made in-game. Shipsets can only be made in XML.
I'm just suggesting to try a game-made custom faction to test mod ideas first, because they're so much easier. If something doesn't look right, (for example the Bubblegum colour scheme either looks like magic or a child's toy depending on the sort of ship you want) then you can remember for doing the XML.
Once a shipset is done, a mod that shares the same folders will recognise it, and furthermore if the game picks that mod up it means it's all right. Basically shipsets will be selectable in-game as well as allocated to a mod race.