Now it is possible to add Screenshots by uploading a file.
For that you first need to use a different program, take a screenshot, save the screenshot and then go back to TestRail and upload the screenshot.
It is technically possible to include the screenshot functionality into TestRail increasing its ease of use. For example BonFire has this functionality: http://confluence.atlassian.com/plugins/viewsource/viewpagesrc.action?pageId=248873075 (Attach screenshots to captured issues)
When writing test cases and also when filing defects this functionality would come in very handy.