Upload your image as you normally would - by clicking the image upload icon ( picture frame with a tree in it )

Once your image is uploaded and visible in the editor, click on it ( to select it ) then click the link icon ( sideways 8/chain link )

In the link popup window click the "Popup" tab. Put a check in the "Javascript popup" check box, then in the Popup URL field, enter the FULL URL to the image that will "pop up" ( including the http:// and domain name parts )

Enter a size for the image width x height - making it slightly bigger then the actual image is so that you have space around the image.

Put a check in the following boxes:
Show Scrollbars
Make window resizable
Insert 'return false'

Then click "Insert"

Click Save to save the item or if you need to continue on to add Options and inventory click the "Add Options" button.