I've had the same problem with my tablet. Solved it by running my pics through Autodesk's 'Pixlar Express' (Thanx, Travis). The pic may start out looking upright on my tablet, look upright in Pixlar, and look upright after saving (just like the original), but looks correct after uploading. Might be a buried orientation code in the pic file that gets fixed.
Also, I crop, resize and reduce resolution. Can often get a 700kb file to shrink to less than 50kb, which really helps upload/download times...