Jessica replied that you could use Dropbox... huh? There is a Dropbox App (I may be the only one who didn't know this) that will allow you to access your Dropbox files on the computer (where I keep good photos of paintings taken with other than the iPhone camera), save them to your Photos App on your phone, and simply use them to post to Instagram on the phone! Seemed too good to be true... It's not! It works like a charm.
To make it a little more clear for all of us visuals out there, here is a short 2+ minute video to explain it simply. I did just as this guy suggests, and was posting in about 10 minutes from Dropbox.
Love when this works like this...