I use a couple of downloadable programs. They're both free, but I think that they prefer that you to donate some money.
Anyway, the first one I ever started with was the program at Paint.Net. I don't know if I'm allowed to link it, but you can just type in Paint.net in Google search and it should pop up.
The next one was the GIMP program. I like this one a lot. It comes with all kinds of good features already, so you don't have to download a bunch of plug-ins. You still can though. Again, I'm not sure if I can link to it without breaking a forum rule, so just google GIMP in the search bar, and it should show up.
Those are the two that I use. Also, I'm pretty new at the signature and wallpaper making stuff, so hopefully someone else posts that has a lot more experience with this sort of thing than me.
I hope that helps!
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