Hi Kelly,Currently, in Flex, there is not a built in functionality to add images to an individual session. This has been something I have been asking for awhile. I hope to see this functionality as Cvent announced they are revamping Sessions functionality during Connect. A short term fix I have is I added the image sideshow widget above the sessions widget and labelled each image. Not ideal, but it helps present a visual to the registrant.
I hope this information is helpful!-Kevin