There's a bunch of online services that do this, where you just choose the file you want to resize on your computer and it returns it back to you resized smaller. Here's one I just tried and it worked as expected... I put in max width and max height as 600:

http://www.imageoptimizer.net/Pages/Home.aspx

Once you get the resized image back, try uploading it here and it should work. Let me know your results!

DocJohn