I had the same problem earlier. I don't know if its the right way but it worked for me-
-I emailed myself my cell phone pic then downloaded it from my email
-I have a mac and opened it with preview
a. edit -> select all
b. tools->adjust size
c. I believe the max .jpg size is 1600x1200 pixels so i just type it in
-Upload through the attachments link
Hope this helps, if there's anything easier I'm curious to find out too