I don't know why JA has become reluctant to provide the information needed to make best use of their products. Typical is that the designer reports the image size that was used for each design layout as well as the smaller version sizes (and different aspect ratios) to use to optimize for smaller screens.
My simple answer using a quickstart installed on localhost is to go through the images/joomlart folders and check the source image sizes. If you are confused about where things are used, edit each image to overlay a text of its dimensions (and path?). Then, as you see a layout rendered on a demo page, you know how best to replace it. If designing for someone else, leave that example with dimensions on a page that has elevated administrator access requirements so it doesn't show to public, but shows to logged in staff users.
Hope this helps
