Here is a quote from Miss. Brassheart on how to do it for Mac
@ Jacoby: Do you have a Mac or a Windows? For Macs: open the picture. It should be shown in a program called Previews. Look up to the very top toolbar on the very top of your screen. Click Tools. Find Adjust Size and click it. A window should pop up with a bunch of tagged numbers. Make the number tagged Resolution smaller until the new size (shown at the bottom of the window) is less than 500 KB. Hope this helps! ?
Just to note: The file size limit is now 2048kb rather than 500kb