GIMP looks intimidating when you first run it, but don't let that scare you. You are probably only going to use 4 or 5 functions to start with, and there are lots of instruction for GIMP available on Youtube and other places. I got a book from my local library. Take them one at a time and it's not really that hard.
What you must learn:
Tools - Select, Color Picker, Move, Text, Bucket Fill, Pencil/Paintbrush, Eraser, Color selector, Opacity (for GIF and PNG type images)
Image - Scale Image, Crop to Selection, Flatten Image
Layer - Scale Layer
Colors - Auto>White Balance, Brightness/Contrast
File - Save (saves your work progress, not your image), Export (saves the image itself)
Layers and Filters are really powerful, and you can do some really neat stuff once you learn the easy stuff.
You can create a signature with MS Paint, but it's not as easy once you learn GIMP.