Problems inserting Image Pop Up

Hi SI Team

Please go here and login:

http://finalbug.net/my-profile-here

This way you can view the following post (registered users only) :

http://finalbug.net/blog/entry/502-all-you-need-to-make-your-movie-for-free-02

Please try and insert a Pop Up Image for the second image.

See the attachment below.

I can not do this and am wondering if this is just me or a bug.
I have got this to work before eg here:
http://finalbug.net/blog/entry/490-all-you-need-to-make-your-movie-for-free-01

Blog Images are 775 Wide and the pop ups are 1280 wide.

thanks in advance

Paul