Basic image editing - multiple files

I have 250 small image files (jpeg/png - originals/thumbnails)
I need each image processed & saved in the following way;

1. Replace any instance of imagery with a placeholder (see example) in the same dimensions.
2. Replace any button colour with #25acd6 (or as close as you can get it).
3. Save (as same extension jpeg/png etc..).

Attached is a basic example (medium & thumbnail version)