We wouldn’t normally refer to images as “MIME content”. They’re images, or more broadly binary content.
MIME is a method of encoding binary content for contexts such as SMTP where raw binary transfers are not supported end-to-end.*
Marketo does not offer a means of generating binary content, nor should it be expected to. For that you’d use one of the many Adobe design products!
* You can create a private SMTP infrastructure which supports the
latest binary extensions but this is not possible over the wider internet.