Images purchased from a stock photography site (see my article) are almost always provided in a JPG format. JPG does not allow transparent areas in a picture, so when you slap it on your slide, the background will cover up anything underneath. If your slide background is white, you can usually find images with a white background. These can merge in smoothly if you use the PowerPoint Format-Send To Back command. But if you have a slide background with a different color or need to overlay multiple images, you’ll require additional photo manipulations to make it work.
The past few versions of PowerPoint have included a command called “Set Transparent Color.” This is very easy to use. You click on your image and click the command in the Format group. Then you simply click on the background color in your picture and it magically becomes transparent. Unfortunately, this process often leaves a jagged outline around your image or leftover bits that didn’t exactly match your color selection. It also only works on solid, single-color backgrounds. I have never been very happy with the results I get.
This is one area where upgrading to PowerPoint 2010 is tremendously helpful. Microsoft added functionality in this release that can make your life much easier. A new command on the Format bar is “Remove Background.” It lets you drag a box around the main subject area you want to keep and tries to guess at the other background parts that should be made transparent. It does a very good job with simple backgrounds such as all black or white with an obvious foreground image that stands out. But you have additional control if PowerPoint guesses wrong. You can use an onscreen pencil to mark parts of the picture to keep and other parts to make transparent. When you are happy with the selections, you finish the process and get a much smoother transparency.
Of course the highest level of precision in making an image transparent comes via the use of third-party image processing software. Well-known examples include GIMP (downloadable for free) and Adobe Photoshop. These utilities can have a significant learning curve, and are not appropriate for the casual presenter just needing to place a quick graphic on a slide. But once you learn your preferred program, you can produce very nice looking results. You will need to resave your image in PNG or GIF format so it can retain the transparency settings. From then on, you can import the altered image into any desired slide.