First add the 'Pop-up gallery' to your page, by clicking 'Insert' –> 'Object' and from the drop-down choose the 'Pop-up gallery'. Alternatively choose the 'Files' icon (in 'basic view') or the 'File Manager' icon (advanced view) and both will launch the File Manager. You can then navigate to the 'Image tools' folder available in the 'Library' and then locate the 'Pop-up gallery' which you should click on.
You can now close the 'File Manager' and move the 'Pop-up gallery' by clicking on it once and then dragging it to the correct position. You can also resize the Gallery if you wish.
Once positioned you can then double-click the Gallery to start editing it. You will then see the Gallery toolbar at the bottom of the pop-up gallery, which will offer a number of icons.
Click 'Add Image' to reopen the 'File Manager', and select one or more images to add to your gallery.
When you have finished adding your images, close the 'File Manager' and go back to the gallery.
You can reorder your images by selecting one of the thumbnails, and using the left and right arrows in the 'Pop-up gallery' toolbar to move their position. You can also use the 'trash' button to delete them.
Double-clicking on the image will open the full size version, which also happens when viewed by your visitors. You can also add a caption to the bottom of the pop-up beneath the full sized image. Just click back on the Gallery to close the pop-up.
The set-up and style panels in the SiteMaker Editor, allow you to change some of the more advanced settings on the 'Pop-up gallery', e.g. the number of images displayed, the pop-up size, aspect ratio etc.