Its much easier to do on fb via laptop rather than fb on mobile, but if you need to do it on a mobile device then open fb in the internet browser rather than using fb app....
Make sure the pic is full screen, and right click to get a link.
I could not post pics either until Esther explained how. If you search Tack Room for "technological dinosaur" you can see my attempts and explanation. Hope it works for you.
I manage to post using imgur from my iPhone. All I did was download the app. If you tap the image once then click the share button click copy link and post it between IMG tags on here.