You can customize what you want to index by using conventions in EPiServer Find

//using EPiServer.Find.Cms.Conventions;
 
ContentIndexer.Instance.Conventions
  .ForInstancesOf<LoginPageType>()
  .ShouldIndex(x => false);

...preferably at application startup or similar. So add conventions like above to remove the content types you are not interested in getting search hits from. 

You can read more here...
http://world.episerver.com/documentation/Items/Developers-Guide/EPiServer-Find/11/Integration/episerver-7-5/Indexing/

Also check out the Alloy template example website for implementation example. For images you can insert your image type you want to exclude from index in below like

//using EPiServer.Find.Cms.Conventions;
 
ContentIndexer.Instance.Conventions
  .ForInstancesOf<ImageData>()
  .ShouldIndex(x => false);

Hi,

Thanks for your reply. This works fine. 

So what is the way to make Find not index ContentAssesFolders/image files?

You can use the same method like above but insert your image type instead (ImageData or similar). That will tell Episerver Find not to index those. You might have to trigger the scheduled job to remove the images that were already indexed btw...

@Aziz: If you want to exclude special folder from being index you can go:

ContentIndexer.Instance.Conventions.ForInstancesOf<IContent>().ShouldIndex(x => !x.ParentLink.Equals(siteSettings.contentAssetFolderToIgnoreContentReference));

siteSetting could be start page of each site!