If you want to make your own images, you should find an image editor that supports the functions of pixel editing while also respecting the palette. I use mtpaint; it's decent but gets a bit unnerving at times. Several people say photoshop works great. Some say they use gimp, however you'd have to figure out how to get it to respect the palette because when I try it, it sets it to the default and ruins the image. Aseprite might be the best tool out there for this work, but it costs money. It's a dedicated pixel editor with many functions for exactly this kind of work, and it's rated really high among users or so I hear.
Once you have a decent pixel editor, you can set out sketching designs, trying your hand at pixel editing, making tweaks to existing work, or perhaps trying to convert higher detail images into small pixel sprites. Whatever you do, you'll be gaining experience while also figuring out what works best for you.
If you can come up with good rough sketches, I can help you fill in some of the details, however the amount of effort I'm likely to put in is low. Still, I like a decent pixel project from time to time.
In Falko's tools there is a HWP tool that can help you make tanks. If you can figure out how it works, maybe you can teach me. I am trying to make some modifications to tank appearance. If you're interested, I'd like to collaborate with you on making some cool tank art.
