1) After you add your gallery to a page, click on it again and select "custom template"
2) Select one of the 9 custom templates
3) Click save
4) Exit edit mode and publish edits.
If you are adding a large gallery to a page, the first time it loads there may be a slight delay as it generates thumbnails for each image.
CDN (content delivery network)
Bootstrap Image Gallery comes with built in CDN functionality for CDN services that provide you with a URL.
Simply enable the CDN option by setting it to true and set your URL for your CDN.
Your images will now load from your CDN instead of your own server