Taskbar Thumbnail Tweaker is a free application for Windows 7 that enables you to modify the behavior of these previews.
The program allows you to change the size of the thumbnail previews, with options from 200px up to 1000px. You can also change the time it takes them to appear, from instantaneous to up to a second of delay.
Another option is to get rid of the preview image, leaving just descriptive text in its place.
If you know that you don't want to bother with the previews, you can also use Taskbar Thumbnail Tweaker to disable the function on your system.