Go to the site's media library to get it. After opening the media info, right click on the image to find the image URL. (In Chrome, you can click "Copy image URL" or "Open image in new tab.") Copy the URL up to the upload date. Don't include the upload date or the slash at the end. For example, if the image URL is...
Then the Upload Path is...
Here's where I ran into trouble again: Looking at the Site Settings, I saw that there might be any combination of the fields "Upload Path", "Upload Url Path", and "Fileupload Url". Depending on when the sites were created (I think), the Settings might include 1, 2, or all 3 of these fields, and NSM seemed a little inconsistent on which field the plugin would check. (Maybe the Plugin Author could shed some light on this.)
I believe you'd be safe updating all of these fields, but it couldn't hurt to write down what's originally there in case you need to revert it later.