I had the same when I first joined, ...usually says the picture is too big no matter how many times you make it smaller.
Best thing to do is use Photobucket. If you don't have an account it is free to use & join plus it's pretty handy for storing pictures and posting online.
Guide here: http://www.350z-uk.com/topic/53047-how-to-post-pictures-via-photobucket/
Photobucket here: http://s273.photobucket.com/