If you intend creating or editing bitmaps for skinning wireframes or for creating backdrops, you'll need a graphics editor. Again, you've got the choice of free programs or ones you have to pay for.
The Gimp - GIMP, sometimes referred to as "The Gimp," is a freely available open source application for created and manipulating graphic images that runs on Linux and other UNIX-based operating systems. GIMP is distributed under licensing terms defined by the GNU project. You are likely to find GIMP as one of the optional applications that come in any large Linux package such as those distributed by Debian and Red Hat . You can also download it directly. GIMP offers photo retouching, image composition, and image authoring and is favorably compared by users to Adobe's Photoshop and Illustrator applications.
Paint Shop Pro X - Does everything you need it to. Plus there are loads of tutorials out there, for example our own animated tutorials.
Loads cheaper than Adobe Photoshop, but has pretty much the same functionality and most people reckon it's more intuitive. They used to do a student/teacher version but they don't seem to do it anymore. More than adequate for creating bitmap textures etc.
Adobe Photoshop - Adobe Photoshop CS2 software, the professional image-editing standard and leader of the Photoshop digital imaging line, delivers more of what you crave.
Groundbreaking creative tools help you achieve extraordinary results. Unprecedented adaptability lets you custom-fit Photoshop to the way you work. And with more efficient editing, processing, and file handling, there's no slowing you down.