I go to the full site on my phone (Galaxy S5 Active)...go to thread to "REPLY"...hit "go advanced"...then in additional options, hit "Managed Attachments".....It will take you to upload Manager....hit "add Files"...it will take you to phone files....pick picture (1 at a time)...hit "Upload"...it will pop up on upload Manager...then hit "done" at the bottom right...it will take you back to Reply screen to finish!
Does not take long when you do it a couple times!