How to reskin

How do I reskin textures?

I want to reskin a poster so it looks like one of the CEDA ones in L4D2.

Edit: Title fail - should be “How do I reskin?”

You need: Somekind of free photo editing software like: Gimp or Paint.net and if you want to pay: Adobe Photoshop or Paint Shop Pro. And also VTF plugin.