I don't quite understand your situation. I Have Avast Mobile Security Pro AND Avast Cleanup Premium. When Avast Cleanup is installed, it deactivates the Safe Cleanup in Avast Mobile Security Pro as Avast Cleanup offers more features. So the Smart Cleanup in my Avast Mobile Security Pro is greyed out and turned off. However, my understanding is that it does not offer a function to do automatic cleanups even when it is working. Even basic Avast Cleanup does not offer automatic cleanups: only Avast Cleanup Premium has an automatic cleanup function. So, if you do not have Avast Cleanup PREMIUM, Avast did not and cannot have automatically deleted your photos or anything else. Either you pressed to start a Smart Cleanup, then pressed again to delete the recommended files found for cleaning after the Smart Cleanup ran and showed you its recommendations. Or some other app did it. I actually have 5 apps on my phone that CAN do cleanups and deletions, including of pictures, from the cleanup app from my phone manufacturer 'Super cleaner' (which is terrible - it lists far too many things that are NOT for cleanup, so I never run it) to the Google 'Files' file manager App. I went through all and configured no automatic running, and no notifications from most of them, and just use two, including Avast Clesanup Premium, which only run when I tell them to, and I check what's offered up for cleaning after their scans before ever letting them actually delete anything.
If you do have Avast Cleanup Premium, the setting for automatic cleaning is in the menu (three bar icon, top left of main screen) as the first item in the 'Cleanup Premium settings section - tap it and check it is set to 'Off'. It's also possible to configure Avast Cleanup premium to upload photos to dropbox or Google Drive, then delete them from the phone. That's not default behaviour, but if you've been randomly changing settings in Avast Cleanup Premium you migth have set that, in which case your photos would be on your dropbox or Google Drive.
If you DON'T have Avast Cleanup Premium, then it's likely some other app with a cleaning function on your phone you need to look for.