For a text based Search Bot, images are a break or hole in your pages content. As a result it doesnt know what is in your image and how to fill the hole. Therefore, Alt tags are the single most important SEO attribute for your images. It is imperative that you use this opportunity to provide a contextually relevant description of your image content. As described by Google, a good example of an alt tag is one that is short, sweet, and informative. It does not contain Stop Words and has not been Keyword Stuffed.
In order to set the file name of an image in Stacks, simply double click on the image to enter edit mode. There you may define the Alt Tag for the image via the page inspector. Please note that all images on the same page should have unique alt tags! SEO Rx will detect and warn you if you re-use an alt tag on a page.