I think there are several reasons why listings aren't posted instantly to the Internet.

The first reason is the agent that takes that listing. Most agents in my association depend on their office to input their listings to our MLS. Sometimes they turn them in the next day, some times two or three days later. Our MLS rules allow 10 days to get a listing into the MLS from the date the seller signed.

The second reason is the technological limitations of the Association that inputs the listing. Strange as it may seem there are many boards and associations across the country that are pretty far behind the times technology wise.

If an agent has a web site it is possible for them to post their own listings instantly on the Internet but the average Realtor is still pretty far behind the curve with the technology and ability to do it.

Another reason is several of the big listing sites like Realtor.com and CyberHomes only pull data (listings) from boards or association. Individual agents can't post listing directly onto these sites which creates a time lag. Speaking from personal experience I know that Realtor.com only updates our boards listings weekly and photos are only sent to them from my board bi-weekly.

Lastly there are lots of listings with no photos. Why? Because someone has to physically go to the house, take a photo, and then deliver it somewhere, either physically or electronically. Again using my board as an example they just developed the ability to receive pictures transmitted via e mail in the past six months. Before that you (or the MLS photographer) had to go to the property, shoot a film photo, get the film developed, and either deliver it in person or mail it in. That in itself caused a huge time lag.

Eventually the majority of listings will be posted instantly to the internet but eventually is still several years down the road in my opinion.