I'd say the easiest way to do this is not to use an Image block to output the image where you will then have to wrestle with CSS to position something over the image, and instead use the Container block where you can then specify a dynamic image as the background, and you can then add content within the container (e.g. a button or single field) which you can align as required and adjust the margin or padding to get the result you need.
I suggest you try it, and if you get stuck, as for help with details of which part you are stuck with.