So how can you make the slides look good?
PowerPoint slides and formatting
When you include PowerPoint slides into the slide-deck for a service, they pick up the formatting that is used in the file you are putting them into. So you can use the same slides with your own color-schemes and background images, with a minumum of fuss. You are not limited to the style, images and colours that someone else has chosen, which may not suit your church building, congregation or purpose.
But to make this work, you need to use PowerPoint's "include" or "reuse" features, not copy and paste. How to do this depends on the software version you are running: LiturgyTools.net has instructions for PowerPoint 2003 and PowerPoint 2007.
You can edit the overall formatting for all the slides in a presentation from the "slide master" view. From here, you can set the background image, font size and style, and the color scheme used.
If you want different slides to have different backgrounds, you need to either create more than one master slide (and make sure you apply the right one to each slide), or don't put any background image into the master side and just set the background of each slide individually (right click on the slide and choose Background)
Where to get pictures to use with your slides
There are many commercial services that sell suitable images. LiturgyTools.net, also has suggestions about finding copyright-free pictures that can be used as slide backgrounds.