It's not easy to spot but still there! I'm using a laptop for this below, not the app it may w, ll be different on that device which I do not use.
On your messaging page, to the left where you start to write your message there's an odd looking icon an L
'Holding a Square' inside there are ' Peaks / mountains ', click on this and you can add your photo from there.
Sorry for the poor description cannot describe the icon any other way! 🤔
