To decrease the size of the picture in the photo tool I was using, I found a properties panel with a compression tab. I went to the resolution tab, decreased from 200DPI X 200DPI, down to 100DPI X 100DPI and set for low resolution. It compressed the file a lot. I don't know if it would adequately compress one that large, but it's worth looking around in your photo tool for a properties selection somewhere.

Best of luck,