When you upload a photo to your MLS service, that service may do a number of things to your photos including adding a logo, resizing, etc. And then, although it may seem convenient, the MLS service will syndicate your photos from the MLS to other websites like Zillow and Realtor.com. Each one of these steps degrades the quality of your photos, but it's easy enough to fix.
Fortunately, you can change the photos on Zillow and Realtor (and other sites) yourself. By uploading your photos and not relying on the copies of copies of copies that are syndicated by your MLS service, you can be assured your photos will be displayed with the highest quality on the popular real estate search engine sites. On each site, realtors have the ability to manage their listings. Below are some links to manage your photos (and listings) on Zillow and Realtor:
Zillow Photo Management Links
Realtor.com Photo Management Links
A couple easy steps in managing your photos can make the difference between buyers passing on your listing or falling in love with it. Quality photos get quality results.
If you need any help with this or have other questions, give me a holler any time.