Currently, I use the default.yml file blueprint with a couple a tweaks, and generally it works well, but when I upload an image to a page, in order to set attributes like altcaption etc. the Panel loads a new page. So I think have to make the changes, navigate back to the page editor, and find my place in the post to resume editing.
I’d much prefer to have the image attributes appear as a modal, like on this image but I can’t work out a way of doing this. How would I go about setting something like this up?
That’s not possible (at least not out of the box), in the example you show, you are in the context of a block, where the caption is set for this block specifically, not set as meta data of the file.