help! pop up images

Oh no.
What's wrong with me? :frusty:
Please tell me if I did everything right.

I saved 3 sizes images. The small ones, with the simple name (no suffix) in the main images folder. The _MED in the medium folder. The _LRG in the large folder.
I manually uploaded from the admin panel the medium image for the product description and the small one for each category description.
I don't know where to upload the LRG. I just put them in the large folder via ftp.
I set the size of the JS popup window to a slightly bigger size than the large image.
What else should I do?
Why do the popup window initially opens as a small image then immediately becomes as big as the medium one and then displays the MED image?

Any ideas?

Can I control somewhere the url of the image displayed in the popup page?

The website --> http://www.testingdomains.co.uk/shop/

Thanks a lot

Francesca