how can I bypass the screen lock at startup on the imac

I understand now. Your wireless keyboard should work to get you into safe mode. Press and hold the shift key immediately after you hear the start up chime and hold it until you seethe grey Apple Logo? If you press shift too early or release tool ate it won't work.

If you mean the log in screen I believe in the Accounts preference pane you can enter your password and select auto log in.