Was it side ways on your computer BEFORE you uploaded it? If it was, what application opens it up on your PC?
If you have Microsoft Office installed, one of the parts of that is called 'MS Picture Manager'... if you have that, you can open up the pic and click on the roatation button then re-save it and upload it again.
/edit:- Would this happen to be the offending picture?!
If so, just save this version and re-upload here.