How to access the raw content of a views field, for use with Services Views
You can do it using relationships! Under Advanced, add the relationship File Usage: File. Then go add a new field, and you'll see File: Path.

Alternatively, you could use the Image URL Formatter module. This allows you to print an image as a URL. Then in the field settings in Views select this under 'Format' for the image field. The module has some features that would be harder to achieve with the relationship, but for most cases the relationship should work.