When building a view, the image option under "Channel elements: core" states the following:
Path to the image to be used as the artwork for your feed, for example sites/default/files/AllAboutEverything.jpg. Allowed image formats are GIF, JPEG or PNG. Maximum image width is 144 pixels, maximum height 400 pixels.
If I enter an image path of "sites/default/files/AllAboutEverything.jpg" and view the RSS XML, I see the following (tags replaced with brackets for ease of reading):
If I then run the RSS feed through the W3C Validator Service (http://validator.w3.org/appc/) I get the following error message:
This feed does not validate.
line 9, column 101: url must be a full and valid URL: /sites/default/files/AllAboutEverything.jpg [help]
So it seems like it needs to be a full path URL, but does not render as such. If I enter the full path URL into the field, it renders with a slash in front.