When you select your gallery (choose gallery) and select a new design (choose design) new code is produced that needs to be placed in your web page where you want the gallery to appear. The new code can be found in the 'Add to Site' tab.
How that code is added to your page can be done in different ways; either your web designer would need to just paste the new code into your page, or else she could create a data source (i.e. a MySql table) that contains a record for each of the code blocks for each gallery type, and an admin page in which you could apply your selection.
Of the two, the first would be easiest, especially as your designer might have to make changes to your page to accommodate a new design's size and placement.