How can I pull slideshow filenames from the database using jQuery?
Here's another option - how about create a view to generate JSON for the image paths and then pull that into your JS?  

The module "Views Datasource" can output your view as JSON.  You would need to create a view that pulls the images from the content type you are using and make sure that the image is rendered as a URL (rather than the actual image tag).

From there you could update your JS to pull the JSON from the view output.  Create a loop that sets a variable with the JSON data (as a comma separated list) and use that variable within your backstretch function.

I can't provide any code examples at this time, but hopefully that can get you started down a path.  Here is a stackoverflow article that provides examples on how to convert JSON to CSV and store that in a variable:

https://stackoverflow.com/questions/8847766/how-to-convert-json-to-csv-format-and-store-in-a-variable