CSS to increase image size when using a profile matrix question

Hi,
I am using a profile matrix question with images. The images are 120px x 120px. When I insert them, regardless of the size, they become very little. Is there a way that I can modify this so that the image fills the whole grey square? or make them bigger?
I need the images so that the survey responses are accessible to screen readers. But if they images are too small it just won't be suitable.
any tips?
thank you.