From 1.X to 2.X the system will create a 2.X database and then add those images to it. It will also move the old images to images/old and it will add an .htaccess rule to keep the old links working. Then from 2.X to 3.X the system indeed do everything from scratch but... Chevereto 2.X table.chv_images is pretty similar to Chevereto 3.X table.chv_images and you can even run the query from the command line or MySQL, you actually don't need to run the upgrade tool.
As a matter of fact the thing is just inject content to a website isn't? In that case my recommendation is that you should took the code of the 1.X to 2.X upgrade and craft your own script to do the job. As you may see the source is pretty clear, I just did a directory iteration over the image files and one by one I add those to the 2.X database. For 3.X is the same logic, iterate a directory then inject those content to the existing installation.
To be honest I can add an importer like that but that will be several releases later because it not in the immediate priorities list.