Okay a step by step.
Insert the small image file in the normal way so you have (no spaces of course)
[i m g] www . isp . com / photo_small . jpg [ / im g ]
Then hightlight that whole line and click on the insert link icon (the globe with the chain) and paste the URL of the large image in the box and you will end up with (no spaces again of course)
[U R L="http:// www . isp . com / photo_large . jpg"][i m g] www . isp . com / photo_small . jpg [ / i m g ][ / U R L ]
and when you click on the displayed photo_small it will cause photo_large to open in a new window.