There are a lot of places that we use and suggest acquiring stock photographs and imagery for presentations. One of them is your own digital camera or scanner. You are your own greatest source for images that YOU want to use, that YOU own. Don't be afraid to take your own photos and use them in your presentation. It adds a lot of authenticity to the production, and really brings a lot of connection to your audience.
Another great source for free stock photos is StockArch. There is a good selection of really nice photography to use as you please. We also use iStock for low-cost photo and video clip options, and a few others found on the web, too. Don't be fooled into thinking that you need to use high-cost stock photos from Getty or other stock photo houses (are there any left that Getty didn't buy up?). There are plenty of other options that won't cost you much at all.