Can I wrap a text around an image in a free form or html content block in SFMC Content Builder? I would like to have white text (i.e. unsubscribe, preference center, some small general data on thje sender) on a blue-colored image as background. If it is possible, what would be the correct html/css code for that?
This question seems more relevant to general HTML email development than anything specific to SFMC. You'll need to use VML in order to ensure the background renders effectively within non-supported clients (looking at you Outlook).
Here's a great site for generating this code. Just add your text within this to accomplish what you need.