First impressions seem to matter most when it comes to dating, job interviews…and real-estate listings.

Researchers tracking the eye movements of subjects who looked at online home listings found that more than 95% of users viewed the first photo—the one that shows the exterior of the home—for a total of 20 seconds. After that, their eyes tended to flit all over the screen, according to Michael Seiler, founder and director of the Institute for Behavioral and Experimental Real Estate at Old Dominion University at Norfolk, Va.

“Without an eye-catching photo, the battle is lost before it begins,” Prof. Seiler says. “You have to grab people’s attention within two seconds. Do it the way a billboard does.”