Steam Notes is a feature that was added in June 2023.
It makes it possible for players to write their own notes inside of Steam Overlay(accessible via Shift+Tab) and pin them wherever you want.
What makes them even better is that players can add their own images, which is what this guide is about.

1. Alt+Print Screen
   It takes a screenshot of your active window and saves it in clipboard.
   
2. Win+Shift+S
   It's a snipping tool built in Windows. It allows you to select an area and take a screenshot of it. It is also being saved in clipboard.
   
3. Ctrl+C or RMB+Copy
   Supposedly this method works the same as others as it "saves the image to clipboard", but I couldn't make it work. I'm not sure why, but I have a feeling it doesn't save the image to the same "location".
   Let me know if it works for you.

1. Open your game.
   
2. Open Steam Overlay(default keybind is Shift+Tab).
   
3. Click the Notes icon on the bottom of your screen.
   
4. When the Notes window is active, press Ctrl+V to paste your last saved image.

Yes, you can.
You can either save the image and paste it in the same note OR you can create a new note and paste the image(s) there.
Unfortunately you can't have multiple notes on screen at the same time. So you'll always have to open the overlay and change/scroll through the note.

Most likely you didn't save it correctly or the image is too big.
Sometimes it takes a second to load.
One of the indicators, that it's loading is your caret(text cursor) stops blinking.

This is me using an image of Dak47922 map with 30% transparency.