You can use PowerPoint as a graphics program — and many people do. It’s not high end, but if your needs are simple, the fact that you know and have it is a great plus. PowerPoint 2007 improved the graphics engine and you can add many more effects, such as soft edges, beveled edges, and soft shadows.
You can insert photos (Insert> Picture> From File or Insert tab> Picture), then crop, resize, format, and move them. You can use AutoShapes/shapes as well.
When you have a look that you want, group everything. In PowerPoint 2007 and later, press Ctrl+G. (In PowerPoint 2003, go to the Drawing toolbar> Draw> Group.)
Finally, right-click and choose Save as Picture. You can save in a variety of formats.
You can then use that image in another program, to upload to a website, or to reinsert into PowerPoint.
Twitter background created in PowerPoint
Here you see a Twitter background that I created in PowerPoint. (Please follow me on Twitter for additional tips that I don’t publish on my site. Go to www.twitter.com/EFinkelstein.)