Get GIMP or Photoshop (I use GIMP myself), they’re both great programs, the only difference is that one is free and the other is… Good for pirating. The other tools you need are already provided if you got Grim Dawn installed, you’ll simply need to launch the asset manager, create a mod, this kind of thing. So I guess just get yourself GIMP, find where the texture files are hidden after extracting the whole pack of files (probably resources\items\gearhead), create a copy of the one you want to change (it will export in psd; using the texture viewer - which you can edit with GIMP), and then it’s annoying stuff I’m too lazy to write about because I don’t know the process by heart. I suggest looking around the tutorials and help section for that.