I am new to digital photography and cannot find a solution to my problem. If I check the compact flash in my Canon XTi it says no images, but if I put it in the card reader and then into my laptop, there's images. Any ideas?

Check the contacts in the camera. Blow in the slot (or use canned air) you could also take a touch of rubbing alcohol on a q-tip (not wet! Just a smidge) and insert it into the card slot to try to clean the contacts. If it doesn't work, then contact Cannon