You guys are morons.
He can't help it that he doesn't know what GFX is, and you can atleast explain to him that GFX is a synonym/abbrevation of graphics.
You can use Adobe Photoshop for editing images/making signatures and such, it is the most used application for it, but you'll have to buy it or get it from a warez site.
Applications like GIMP are free, or just use MSPaint.