I think it might be the preference settings. I assume you are using Outlook 15 for Mac? There are a couple of ways to do it: 1. Drag the screen capture from SnagIt's history panel to your Outlook message. Or 2. Select the image in the SnagIt window and then Paste to your Outlook message.
I don't believe either of those will work if your preferences aren't set correctly. Your e-mails should be set to Compose messages in HTML by default. This setting is locatedin the Composing dialog box.