Hi Katrin,
Images are not associated automatically to tokens, like e.g. fields are. They have to be set manually in programs (or folders) in Marketing Activities. These are the so-called program tokens starting with "my."
https://community.marketo.com/MarketoTutorial?id=kA250000000KysJCAS
Imagine you had a folder called "German programs". Place a token like {{my.header-banner}} here and give it the value of most used banner image like "http://lp.yourcompany.com/rs/yourcompany/images/German_Header_Banner.jpg".
(That image doesn't need to be hosted in Marketo, it can also be hosted anywhere else on the web.)
You can now place that very token on all emails and landing pages below this folder, and if you want to change it on all assets (without the need to re-approve your assets!) you just have to edit this one token.
That would be a folder token and all folders and programs beneath it inherit its value.
Similar to that you'd have the above mentioned token for {{my.webinar-speaker-image}} in your webinar program and use the token on all emails, landing pages and thank-you-pages that should have that very speaker image.
I'd suggest to dive deep into the concept of cascading tokens, which is - at least in my opinion - one of Marketo's greatest features.
Hope this helps!
Best,
Michael