Juicebox uses certain CSS z-index values for gallery elements to ensure that they are stacked in the correct order, for example to ensure that captions are stacked above images (and not obscured behind them).
The solution to your problem is to ensure that the z-index value for your menu system is greater than any used within the gallery itself so that your menu is always stacked above (and displayed on top of) the gallery.

In your web page's 'style.css' file, scroll down to line 71 and change the z-index of .nav-principal li to 9999.
Incidentally, you currently have two different z-index entries in this block (set to 10 and 298).
Just use one z-index entry and set it to 9999.

Your .nav-principal li block should now look like this:

.nav-principal li {
  display: inline-block;
  margin-right: -4px;
  position: relative;
  padding: 10px 20px;
  font-family: 'Playball', cursive;
  color:#fff;
  z-index: 9999;
}

If you do not see any difference immediately after making the change, then try clearing your browser's cache before reloading your gallery's web page.

This should hopefully resolve your problem.