Hi Simon,
By opening the ‘Appearance’ tab within your image, you can change some of the image its properties and even add one or multiple custom CSS class(es). For example:
For clear instructions on how to create your custom styling within a Mendix project, I would suggest you to read the following guide:
https://docs.mendix.com/howto/front-end/customize-styling-new/
There are also a couple of Learning Paths available that can help you get up to speed with custom styling.
Hope this helps.