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: Grid Lines and Snap Alignment
Grid lines are a versatile tool that can be used for lining up components on a page. You can turn on the Grid lines by clicking on the Advanced button and the clicking Guides and Snapping. When you select the Grid lines option, it will activate the slider that allows you to make the guides smaller or larger. 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.
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.
If you wish items to automatically align with the guide lines, you can go to the Advanced option, and select Guides and Snapping. Then check the Grid Lines option box under Snap Alignment.
Snap alignment allows an object to be more easily positioned in alignment with the page center, other objects or Grid lines, if those are selected. When an object is moved close to one of these it will automatically jump to an exact position in alignment with the selected objects. Just click the options you want to use as guides.