Step 2 in the process of crafting a sermon with PowerPoint: search
I search for dozens more images than I use, and I download 2 or 3 times the number of images that actually go into a presentation. The more images you can look at, the better your final presentation will be.
If I don't have a specific image in mind I will start with Google to see what keywords bring up. This will usually get me into a ballpark for ideas and I can start refining from there. For example - on Sunday I preached about the armor of God. I started searching for 'armor' and 'ancient roman armor' to get some ideas. I ended up finding pictures of costumes and medieval armor and video game screenshots. Not exactly what I was looking for.
Tip: keep trying different searches to narrow down your results - move from more general to more specific.
As my searching progressed, though I found some images of roman formations that sparked an 'aha' moment. The roman testudo formation allows the soldiers to stand in the face of any assault - that is the perfect example of Paul telling the Ephesisans to "stand firm" in the armor of God.
Note: Be sure to check the license of the images you get from a Google search. Not all of them are legal to use, so be careful and ethical.
I usually create a folder with the same name as my sermon, so I can find images later. Then I save the images that might work into that folder on my computer. That allows me to try several potentials to get the right fit. Also it allows me to do all of my searching at one time and then to do all of my design later. It just works better for me to divide up those tasks rather than jumping back and forth between searching and designing.
Note: check the links on the right for several good image websites.
Once you have a good idea of what you're looking for, it can be a good idea to move away from Google to something with a bit higher quality. Google yields a lot of results, but most of them are small images - anything below about 500x500 pixels will look terrible when projected. Go to a stock photo website to get higher quality photos.
Tip: you can refine your Google search to only return large photos to get images that will be better in a presentation.
I have had a lot of luck with StockXchange for hosting high quality, free stock photos (they do have some that cost money, so pay attention when you're clicking). There are also some good Flickr groups. And I'm always open to new sites, so drop me a comment if you have something that works well for you.