Click on the Add image to post (bottom left) and then Choose Images (you are now on the postimage website) then Files which allows you to select from your phone - but this is on Android, Apple may be slightly different.
It's possible for me to re-assign individual user names, and no existing posts are lost and all other profile info already entered stays the same, email, password etc. PM me if you wish to do this for a one-off change.