There are two possibilities how to done it:

1. You need to check all the pictures manually and compare them one by one to make sure that all equal and poor quality images were founded and deleted;
2. You can use specialized program to compare images in automatically mode and much faster than manually.

Very interesting solution in such case is Image Comparer from Bolide Software.  

Download Image Comparer

The program has easy intuitive measuring interface can help to organize image gallery in a few minutes with helpful and detailed wizard.

Several easy steps and all similar images with marked differences will be displayed in result menu. Everything that left is only check and delete unnecessary elements.

Image  Comparer from Boiled Softare – right way to compare images fast and easy.

Download Image Comparer

Taking photos or downloading images from the Web is part of our daily life. Over the years, people collect thousands of them in the computer – photos made with the camera, wallpapers, clipart, images and artwork, the list goes on and on.

• Unfortunately all these images are hardly ever kept organized. People save them to different folders in different formats and then re-save their copies to other folders and sub folders. Duplicates take space on the hard disk, slow browsing through the collection and cause confusion as too many times we wind up not remembering where we put that particular photo we need now. Browsing through folders to compare images can be very daunting and would take ages, so many users don’t do this, planning to sort out pictures later. And never do this.
• Fortunately with Image Comparer from Bolide Software, finding duplicate or similar images can be pretty much a snap. Running Image Comparer displays an easy to follow wizard that asks where the user wants to search – within one photo folder or compare images in two different folders. The next step is to choose the folder(s) to scan, select whether to search for duplicates or similar images and set the folder for the output results. That’s pretty much it. Clicking the Process button starts the search, which is incredibly quick.