photobucket.com - an image hosting website. Images larger than 1 MB are automatically resized. You can then further reduce the size through an edit function on the site.
If you are not wanting to reduce the size of the image, but only the resolution - Photoshop should do it. I have never had a problem loading anything on photoshop. You can change the amount of memory cached used by going to Edit>Preferences> then go to Memory and Image Cache.
Good luck.