To any of you who are regularly looking for high quality pictures to use for presentations, I want to encourage you to take a look at Wikimedia's Picture of the Day. You can subscribe to it with any feed reader, such as Google Reader, and they have a wide variety of pictures that could come in handy. They are available in high resolution, so they work well for cropping down to whatever parts you want to use.
I think many preachers make the error in using power point of making it be about the words on the screen. They will put the full text of their sermon on the slides, so that the preaching itself becomes almost unnecessary.
The point of using a visual aid is for it to be visual. Minimize words, and maximize the use of powerful images, such as the ones found here.