Setup a custom template file (essentialy the same as header.php) within the directory:
/blocks/image/composer/banner.php

I've added the image block to the page type "defaults". Checked the "Include block in composer", and selected my custom "banner.php" file with custom dimensions.

The block appears editable when you are editing it in composer. You can load an image in, crop it as you'd expect.

I publish the page, and when viewing the page the block you'd expect to update just includes "Image Not Found".

The actual image gets saved. (you can see it in file manager, and does pull through if you just add a block anywhere, though not using this method with composer...)

Am I missing something?

Regards,
Joe