Question: camera roll white rectangle symbol
I've noticed recently a little white symbol on some of my pictures in camera roll (see attachments). I must say that the pictures on which the symbols appear are normally screenshots stitched together with a third-party app (Stitch It!). I've been looking all over the internet for the meaning, without luck.
Does anyone has an answer for me?
Thank you really much in advance! ☺☺☺
iPhone 8 Plus, iOS 11.2