The sideways issue seems to happen for more and more people. I think that with more and more operating systems about (IOS, Android, MacOS) there are variations in how the info about orientation is stored in pictures and not all devices/software recognise all of the ways of storing this information.
I know pictures my sister takes on her iPhone showed sideways on my Win 7 laptop. However when I upgraded to Win 8 those same photos suddenly showed the right way around. These also show sideways on my digital picture frame.
Some photos I've taken on my DSLR show fine on my PC, but show sideways on my digital picture frame.
Photos I've taken on my Android phone have shown sideways at first when loaded to this forum.
In all the above cases I found that opening the photo in 'Windows Photo Gallery' then rotating them, then rotating them back to their correct orientation again seemed to correct the orientation for all other devices and for this forum. It seems there is a standard windows way of writing the information to the file which everything understands.
I'm guessing based on what I've seen. But it does seem to happen more and more.