Are you adding images by displaying a URL? If that's the case, put the image URLs in a field in your feature/map service and use that field as your Image URL when configuring your popups. Or, if you're creating your feature service from a geodatabase feature class, you can enable attachments on your geodatabase feature class and then add images to each feature as attachments. The limitation with attachments, though, is that the image does not show up in the popup, only the link to the image shows up in the popup.