pictures on the page are not displayed (heroku)

I have a spring boot / vaadin application uploaded to heroku.
it works, but there are stubs instead of links to pictures.
when starting the project locally - all pictures are displayed normally.
as far as I know, heroku has no restrictions on accessibility to external resources. and my problem is most likely from the browser / html side as it seems to me.
from the heroku side, the console does not allow ping.
have any ideas?

upd:
Now I noticed that on heroku the links to images turn into https instead of http. I don’t understand why this happens.
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