I had this same problem too. It took me a while to figure it out - not to mention surfing the web for a bunch of sites all with people writing about the same problem. I downloaded the latest driver, checked all the cables, hit every support website I could find, etc. Nothing worked. Then I did +one little thing+ and it worked. I'm assuming that you figured out the problem by now, it's kind of silly.
When you open *Image Capture* [and the name of the software is the big hint] it projects an image from the scanner onto your screen. But
YOU still have to go to the image and select what you want to scan. When you place your cursor on the image and then click and drag, you're capturing the area that you want scanned. When you do this you will see all the dimensions of the scan area captured in the Scan Setup box to the right of the image. You can manually go in and change the dimensions to match the material your scanning (8x10, 4x6, 8.5x11, etc) as well as adjust the resolution or increase or decrease the size.
Once you get it figured out it works pretty slick. Unfortunately the directions aren't very obvious and the interface on the printer is a bit misleading. Hope this works for you (and anyone else who suffered this issue).