No/some images and blank pages

ok, the issue is that there is not enough time being given to let the images load on the print page. You have two options:

First, add some pause before print time. this can be added in the plugin settings, try a number like 2000 to give 2 seconds.

Second (Better Option), Check out our upcoming version:
https://github.com/baden03/print-o-matic/tree/2.0-redo

This handles the print in a much smarter way and does not require the images to reload for the print page.

Let us know how it goes!

though not every image loads every time

Hmmm… is a lazy load being used for images? Or, maybe the print trigger is clicked before the page completely loads?

The last image (page 13) gets split over 2 pages

page 13! Thirteen pages? Might consider creating a PDF and offering that as download / print… however, if you want to prevent an image from being split over two pages, check out this support thread:
https://stackoverflow.com/questions/2649169/print-styles-how-to-ensure-image-doesnt-span-a-page-break

and finally:

Will 2.0 make it into a Pro version

But of course! We are preparing to release both the free and pro versions togher.

Let us know if we can be of any further support on this.

Have you tried defining this css in a @media print wrapper, like so?:

@media print {
    img {
        page-break-before: auto;
        page-break-after: auto; 
        page-break-inside: avoid;
        position: relative;
    }
}

Actually, after looking at the results, it seems like it is working as expected, you just have an issue with the last image being a bit large. Try adding the following css:

max-height: 100%;