For me, it was Preview rotating it wrong when I saved it to a size that MR would take (under 1.41MB for JPEG). I had to uncheck keep original rotation or something to that effect to get it to keep the vertical rotation. It must be a bug in Preview as the orientation is accurate on my iPhone where I took it.
Hmm, how did you transfer it to your mac? I photostreamed it into aperture from my iPhone. Which btw is awesome, i used to have to email pics to myself.
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EDIT: whoops just got it lol. Umm I took a picture with my cell phone and it seemed to turn sideways and I don't know how to change it to upright.
Was it on an iPhone? Mac or PC?