You could use a normal button with the image, and style it the way you like.
However in my opinion this is more work then should be done, this should be default Mendix. This issue has been brought up before and there are ideas, on the forum to improve it, like: https://forum.mendix.com/link/ideas/228
Another option would to use the clickable container widget. This allows you to set a microflow to be triggered when a div is clicked. This also has the option for a blocking progress bar. You would create a container on your page, add the image inside the container, and add the widget inside the container. Then when the image is selected, you can trigger a microflow with a blocking progress bar.
You could place an image, without an on-click microflow, and a copy of your first text button, without any text. Then simply use CSS to make the new "text" button cover the image (intercepting clicks on the image).