Most photo software packages have some way to view the properties. Also, once you have uploaded the picture, double click on it and you will see the dimensions underneath it. The sample I have is kind of small, but you can see the size underneath it. You want to be less than or equal to 640.
I just have to say that I looked at Jordan's photo again this morning & I can just see the devil in his eyes. I swear I should have waited longer to name him, he should be Diablo