There are at least two things you can do here.
(1) You can use an image for a level and you can specify the pixel height and width of that image. For example: <img src="[% GraphicsPath()%]smiley.gif" alt="Smiley face" height="42" width="42"> But if that doesn't work for you because the image is larger than the available space in the discrete choice table, then you may want to use option 2.
(2) This option presents a thumbnail image which, when you click on it, opens up a larger image in a modal window which expands to fill the available space on the user's device. To use this option, go to https://www.sawtoothsoftware.com/question-library
and download the Image Popup module. Then load that zip file into the Question Library of Lighthouse Studio. Follow the instructions on the download page on how to implement it.