How to change image per category or post view

Try it again.
Change which images?

any image. say an image “a.jpg” in the the current theme folder.

Hmm. If you are saying you want certain images, like backgrund or special decorations, to be different in the presentation of different categories, you can do this by creating different templates.
Look at https://codex.www.ads-software.com/Category_Templates

Something like that is possible also for Pages.