First thing you wanna do is download paint.net 4.0.10
I got my paint.net at https://paint-net.en.softonic.com/
Just clicking this link Paint.net does not work. You have to go to a external site so make sure you careful and go to safe site.
Once paint.net is installed watch some videos on how to use paint.net They are all over youtube and are very helpful.

So now that you have watched videos downloaded and installed paint.net, lets get your files.
Go to your file search and look for
C:\Program Files (x86)\Steam\userdata\129211454\760\remote\431450\screenshots
This is an example of one of my screenshots locations.
Now that you found the png screenshot open it. Ta dah there is your screenshot.

Ok now that you have painted your file or added a picture your gonna wanna save it. Open the file tab at the top left of your paint.net. Scroll down til you see "save as", make sure it the same directory as the one you got the screenshot from. in my case.
C:\Program Files (x86)\Steam\userdata\129211454\760\remote\431450\screenshots.
You will see the screenshot that painted but it is unpainted. Just click it and save.
It will ask you if you wanna replace the png with the current png. Click yes.
Well thats it, now you have a painted screenshot, hope you enjoy painting your Alchemist Screenshots.

Note. If you paint your mod feature screenshot you will see it but the public wont. This only works with the screenshots you add to your workshop mod. If you have any questions feel free to ask. Thanks, Wolf