On a Mac, you can create a screenshot of a specific part of your screen. Pressing CMD-SHIFT-4 brings up a cross hair and you can select the area you want to capture.
Screen shots have become an essential part of my workflow. Rather than worrying how import PDF files, web sites, video stills, or other images into my presentation I just snap a picture of them. The same for exporting PowerPoint slides, the images on my blog are usually screen shots.
No more looking for files, no more worrying about file formats. I heart screen shots.