These are "lightboxes". Click them to open, and use arrow key / buttons to page through, type X or click X to exit.
To make a lightbox:
1. On the editor toolbar, click the Insert/Edit Image button. Don't worry about the Latin - it's just a preview of how text flows around your picture.
2. There are several tabs on the dialog that opens. On the first tab, you want to enter: a. The URL of your thumbnail image, and b. alternative text (a caption for your picture). If you do not have a thumbnail image, you can set width or height to 128 here - but it slows down the website. 
Use thumbnail images - Google Picasa, Flickr, Photobucket - they all provide thumbnails.
3. On the second tab, you enter a. the URL of a bigger image to pop up, and b. select lightbox / lightshow / lightvideo on the "Lightbox" list of choices. A lightbox lets the user page through the images. A lightshow auto-advances to the next picture. Only use lightvideo for small videos. (You still use the Insert/Edit Image button for a video - the image you're inserting is a thumbnail, and the "bigger image" 2nd tab URL is that of the video.)
4. Add blanks (use spacebar) between images for a nice margin. Etc. The spacing and centering in the thumbnails above is just a centered paragraph containing two thumbnails with a blank between them.
