iPhotos on Mac mini repeatedly adds unwanted images

Surely you're not trying to run iPhoto, an app that was discontinued 10 years ago? Do you mean the Photos app?

Can you tell us:

1. By "screenshots," do you mean screenshots that you've taken yourself that reappear? Or something else?
2. Do you use a VPN?
3. Do you use "Cleaner" apps or "Virus" catcher apps?
4. What do you see at System Settings>General>Login Items? No login items would be ideal.
5. Where does your Mac's Photos Library reside-- in the Pictures folder of your internal drive, or on an external drive? or on a network? on iCloud drive?
6. Do you have more than one Photos Library?
7. Are you using iCloud Photos to synchronize your devices? If so
8. …Do you have “Optimize Storage” checked in Photos’ Settings
9. …Do you see all your Mac’s Photos  pictures at iCloud.com?
10. …Have you checked to see if the Apple Account is really really the same on the Mac and at iCloud.com?
11. …What is your storage at iCloud.com? At System Settings>[name]>icloud>Storage , my Mac says 745 GB of 1TBB used. What does yours say?
12. What is your storage on this Mac? At System Settings>General> Storage, my Mac says 745 GB of 1TBB used. What does yours say?
13. What does your Library say at the bottom of the Library view? Like this:   

Let us know…