Hello,
I’m not sure, from your description, how you were attempting to add the media but as indicated in the message you might try using Anki desktop.
Give this a shot:
From the browser you can pull up a given card and you should see a screen more or less like this:
From here, the button you want to look for is ‘suggest a change’:
There are some guidelines as to submitting suggestions, and some that apply specifically to images, that you might look at here: 📚 AnKing Step Deck Submission Guidelines
Please let me know if that answers your question, and if there’s anything else I can help with. 