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Its easy to resize images on your Mac. No one will have to scroll and it will take up less space on the server.

First, right click on the image. Then move over to Open With, and click on Other.
   

Scroll down the list of applications that appear until you see iPhoto. Double click its icon.
   

Once the image loads in iPHoto double click it.
   

Now click File (in the menubar), and then Export. On the Window that comes you need not enter anything but a new width. 500 usually works fine.
   

Save the image wherever you like and post it as normal.