Fixing transparency issue on 77 PNG horse pictures (330x300 px)

We have transparency issues on dozens of horse pictures of our horse game Horzer, see some examples in the attached screenshots. ---> we have created a ZIP file with 77 of our PNG horse pictures that have such problems. All these files need be reworked in order to fix these issues. Attached you can find 12 of these pictures ([login to view URL]). Once awarded the project, I'll send you the 77 pictures, they are very similar (same size, etc). I've also attached the 2 background pictures that are most commonly used below the horses pictures, so that you can easily check the result when working on any file.