The easiest way to show images is to Add the image to the Entity (Make sure you create the blob file and assign the image type), then add Detail component in the page you want to use as modal and select the Entity and the atrribute and make sure Image type is also assigned.
Then in the OK Flow select the Ajax Tab and and Select Open Windows and make sure Modal is selected.
Then go to the "modal" page and go to the Layout, then clic on the Detail component and go to the Outline view and expand the tree view and clic on the attribute and go to the Properties tab and asssign a "Style Class".
Then go to your .CSS file and create a Style for the image like.
or Whatever the px you need.