Yes, you can. In the survey or survey package introduction or in the invitation email, use this syntax:
You need to replace /path/to/img with the actual URL of the image.
Example: 
This does mean that the image has to be extracted from an online location, and you cannot upload it from your computer.
You can also add your study logo to the invitation email by inserting {logo} in the survey package invitation text. Place this tag exactly where you want it to appear in relation to the text.
Please note that for surveys completed via Castor Connect, this is not applicable - text and header for survey package invitations on Castor Connect are not editable.