These instructions are for inserting a WordPress Gallery.  For additional features, you can insert a Foo Gallery.

1. We recommend you resize image files in advance on your computer. – Don’t upload full-sized camera files; you’ll run out of storage on your blog account! – If you want nice big images, say, almost filling a laptop screen , try 1200 pixels high. – Note that images in pop-up overlays resize to fit on the screen they are viewed on.
2. Open your post and click the Add Media button (above tool bar).
   If the images aren’t already in the Media Gallery, to add them, click the Upload files tab (center top) and drag in image files from your computer. Checked images will be added to the gallery.
3. Click Create Gallery (at top left). The Insert Media window will change to show just the selected images. This is a good time to add a Caption, Alt tag, Description, (You’ll be able to come back later).
4. Click Create New Gallery, at bottom.
5. Now you can drag thumbnails to rearrange image order, and make these important changes in the right sidebar under Gallery Settings: –  Link to: If you want a lightbox slideshow to launch you must select Media File , not Attachment Page (default). –  Columns: Number of columns in the grid that will show on the page. –  Size: This is the size the enlarged image will be. (Thumbnails will shrink/expand to fill available space, then when someone clicks a thumbnail they can see a larger version). In the example below, I chose full size (these are 460 high x 1400 wide). In most themes, the pop-up images will fit to available space and not overflow the browser window.
6. Click Insert Gallery.

Note that the first image has a caption (it renders rather large in this theme!), and if you click on the thumbnail,  the description shows with the enlargement.