See also Creating Galleries tutorial How do I upload multiple images in a gallery at once?
I can’t find the option to add Image Galleries from the Area icon any more. In the new Mosaic Site Creator 2.2 version we changed the way Image Galleries are enabled. To be able to add Image Galleries to your site now you should first activate them from Advanced Elements. To do so you should enter Admin view of your site, roll over the Control panel [P] icon on top right, select Advanced Elements and check the checkbox in front of ‘Image Gallery’. Click on Update and then you should have the option to add Image Gallery when you roll over the A icon on your site pages.