Resize Photo File

[Arizona Hummerboy]

Ok I did the ms paint thing, I was able to size down the picture, and save it as a jpg. But when I try to post the picture ,it keeps telling me my piture is still to big. The size of my picture is 756x504 and I need to take it down to 700x500 pix I think so I can post it on the Hummer site.

So how do I get the pix size down from 756x504 to 700x500 ? And make it work?