I am looking to develop a simple 2D (potentially isometric) RTS game with a friend. The way it's looking right now, I will have to be doing the graphics for the game. Generally, I suck at this, and was wondering if there was some sort of editor or other tool that can make my life easier. I would be designing units, tilemaps, etc. I understand GIMP is a thing, but it is generally poor at creating graphics as far as I am aware. And MS Paint is simply trash. Are there any sort of paint tools useful for my specific case? The ability to make isometric shadows (even if through an extensions) would be a great feature as well.
I can recommend IcoFx (commercial), which is still available in its last freeware version (Chip, German).
Start with a hires graphics (256x256 or higher), set the background grid to "large" and start painting with thick strokes. When finished, you can create a new image in the same icon and derive it from the large one.
IcoFx comes with many tools, e.g. one for shadows (are these isometric?):
I never created tiles if you mean that things need to connect seamlessly.
On windows I use the same tools to create application icons as I would probably use to create sprite art.
My preferred app is Axialis Icon Workshop, which is commercial and costs around 60 Euros.
My reason for considering pixel art to be the same as making icons is that I tend to do the highest resolution or largest version of an object I will need, and then use Axialis to provide 50%-smaller and 75%-smaller versions.
The main thing I need for pixel is really good single pixel tweak and good alpha blend (transparent cell) features. Once I have my 32x32 character sprite, creating the 16x16 is easy and automatically has the right alpha transparency.
You can try Inkscape. It is an open source alternative to Adobe Illustrator. It has features for Isometric designs and projections.