Similarly to adding almost anything to your site, to add a gallery, you use the add element again. Each gallery you will see will be different from each other. From professional looking galleries to the less so professional. Wherever you choose to place a gallery on your site, always make sure the images are appropriate for the page or your website.
On WIX there is a wonderful SEO tool called SEO Wiz. It makes the basic steps of SEO even easier by offering you suggestions, such as keywords which relate to your website. Once you set your keywords the tool allows you to analyze the keywords. Have a mixture of short-tail and long-tail keywords, as long-tail keywords are easier to rank for. The tool then offers you an SEO plan, which will hopefully start to rank your website.