Image overlay 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.

If someone could supply me with the code here, I would really appreciate it! Plus, being completely illiterate here, if you could break it down as much as possible (what each section does, where to load/save images, linking the clickable part, etc.). If not, I’m sure I can stumble through. I hope this isn’t too difficult, from what I can gather it seems either an img or div is the way to go (not that I’m positive what either is right now). And I just need the simplest/quickest way, nothing too fancy.

Thanks so much!!