Use a computer or change your phone broswer to a computer version. It does not work on mobile version
Visit
https://imgur.comJust because of this reason (having to switch to desktop version on mobile phone as I often write forum posts using mobile phone) I prefer
https://postimages.org/ where it works normally, without need to switch. Another one where you don't have to switch anything is
https://imgbb.com/. For some reason majority of people still prefer imgur, don't know why really.
Imgur has some high quality images which imgbb somewhat falls short of.
And the latter tends to render them smaller after uploading them.