Overlay an image anywhere?

I am a complete beginner here, so please dummy down any information you can provide, and thanks ahead of time!

I want to add a post-it styled image that can be placed anywhere on my site (mainly the upper right region) and appear above/on top of everything else (e.g. the header, menu, text, etc). I’d like to use it to display weekly events, so I’d love for it to be clickable and easy to switch images each week.

Thanks again.
Adam