There are several tools available that you can use to make sure your images and caption boxes are sized or positioned just the way you want them: Alignment Guides and Grid Lines.
The alignment guides are active by default and show you when the edge of one object comes even with another object, or centers the page either Horizontally or Vertically.
To turn off the guides, you can either click on the button, or go to the More Button, and select View > Grids and Guides. Then uncheck the boxes under Snap Alignment.
Grid lines are a versatile tool that can be used for lining up components on a page. Turn on the grid lines and you'll notice that there is a size adjustment bar that appears above the project you're working on. You can size the grid as you like, including lining it up to the edges of the Safety Zone Warning. You can then line up your images or caption boxes to the lines on the page. Once your first image is lined up, the alignment guides can aid you in setting any others in place.