If you are posting other peoples content without crediting the source then yeah that's plagiarism, but from what I've seen from your previous posts, you've always credited the source and made it clear that the photos were not of your own work.
I recently found out that you can "write" a book based on another story and actually get the rewards from it - For instance Romeo & Juliet Shakespeare Retold by Martin Waddell which I find absolutely unbelievable! THAT in my eyes is plagiarism, as is something like Selling copies of famous paintings and simply saying "this is a copy"

Carry on how you was mate, ignore the unintelligible BOTS saying "we think this is plagiarized from somewhere else" and if it helps your OWN conscious perhaps you could look at donating some of your $'s you earn from said post(s) to the original artist/photographer/website/charity or other such thing, after-all, without YOUR posts showing those photos/pictures/art etc MOST of the people on Steemit might NEVER see it, you could be doing the artist/photographers etc a favour by sending people to their websites that might not have ever heard of them before...

Just my two cents...