The ethernet over powerline system or wireless bridge mentioned by hairystuff seem to be the best options for you. For EOP to work effectively, the electrical circuit in your garage and the circuit that your router is on in the house should be on the same circuit breaker, or at least on the same electrical bus in your braker panel.
A cheap and effective wireless bridge can be made by picking up an old Linksys WRT54G router and installing DD-WRT firmware on it. The router will cost about $15 to $20 and DD-WRT is free. Once DD-WRT is installed, the WRT54G can be configured as a bridge, repeater, access point, etc. as needed. It will be wireless G, but that should meet your needs for the camera project.