My preloader doesn't work correctly

This is the fist site I make and it doesn’t seem to work very well. The main problem is that the gallery (“imagenes”) is quite big and it takes some time to load, it has a preloader but it doesn’t appear until 70% of the movie is loaded.

How can I make the preloader appear when I click the button to open the gallery?

My site is:

Tnkx for the help