Dialog cards do not display images reliably in Moodle 3.4
H5P may work find with one browser, on one OS, at one time, but not work at all on another browser, on another OS, at another time. At least, that has been my experience, and the experience of others - from what I have been reading.
I put together an H5P dialog card lesson on bird identification. You can - maybe - see it here.
Just go down to the bottom, and click on the "Bird Identification" link.
With Linux, and Firefox, some images are occassionally missing. They are arbitrarily deleted and have to be uploaded again.
With MacOS, and Chrome, none of the images show up at all, unless you are logged in as admin. This is not the way I have it setup, and not the way it works on other systems.
With MacOS, and Safari, the images also do not show up, I get this error, from Moodle:
In the case of a change of server, check if you have copied the content of moodledata, especially uploaded files.
Also make sure the file name contains no special characters like '#'
You may also get this error if you have moved files after linking to them from the course page.
H5P puts a hyphen in the image names, no other special characters.
The situation with Windows 7 is about the same as MacOS. I have tried with Firefox, Chrome, and MSIE.
When I looked at H5P demos, I was impressed. After trying to use it, I am disappointed. I cannot rely on H5P to actually work.