Attached Link: http://www.hotfreeware.com/2020/2020.htm

Like this. In Photoshop, open your screen shot of your 800x600 wire frame and open your rendered 800x600 image (or whatever resolution you use, just either keep them the same or resize in Photoshop so they are the same.) Then move the rendered image over the wire frame image and select the layer for the rendered image and the erase tool. If you want to see what's underneath the rendered image, you can set the transparency, strenght to less than 50 so that you can move the eraser tool around just right. Then set the rendered layer back to full strength for the final save. If you need a screen shot app, the best one I found is a free ware at this site.