Just run it through TinyPic or http://resizeimage.net/ and make sure the longest side is no more than 1000px and you're good to go - I do it all the time and it's very quick & easy. There are actually a lot of resizing tools online, but resizeimage.net is one of the easiest. You don't need to use all of the options there - just do the following:
Go to Step 1 and upload your image.
Skip to Step 4 and replace the longest side with 1000.
Skip to Step 7 and click "Resize Image."
Click "Download Image" (which will be located next to "Resize Image" after you click it to resize).