Photographs that are NOT mine in Dropbox - and NOT as a result of using a shared computer!

I'm a retired engineer with ageing tech (old HP pc and 2018 phone). I downloaded Dropbox a while ago but hadn't used it until today. I needed to upload some pics I had just taken to my pc and then email them. I opted for Dropbox from a Windows list and connected the phone with a cable. Dropbox uploaded only 25 pics including the new images. Then I spotted an image I did NOT recognise. A young lady, about 25, clearly sitting for a portrait in business attire, white sheet background. Image info showed it was a colour image but displayed in b&w. Taken on 13th June this year, just weeks ago. "saved from Safari", IMG_0380. I've never used Safari. Could be one pic from a company photoshoot perhaps. How it ended up on my old iphone I find deeply disturbing. Are any of my phone images now on someone else's phone or pc?? Fortunately they are all pedestrian and benign. Nothing really private, but the mind boggles.  Before today, the last photo I took on this phone was in April. The spurious image then appeared, followed by the three new images. I don't do social media and make few calls. Internet radio, the news and Google mostly. Phone is too old for many apps. It has to be Dropbox. They will deny it (do a 'Horizon') but this is a massive security issue! VIZ:  my images sent to the cloud and...   someone else's image downloaded to my phone from the cloud. Nice.