The difficulty in determining what type of image goes in each of the categories in the Memories section of the website arises from the issue that there is some overlap. In a sense, the designation "Photos" is ambiguous as many of the images created for documents are in fact photographs. The divisions seem to be more based upon the content of the image rather than the physical process by which it was created. For example, a written document could either be imaged by a camera or a scanner, but in either case, it is still an image of a document.
Of course, it is my opinion, but I feel that the category of Photos includes images of people and places and other items pertinent to the history of a particular family. This is a very broad category and includes anything that the family feels is relevant to preserving family history. Of course, there are guidelines as to what are and what are not acceptable images, but within these guidelines there is great latitude.
The distinction between Photos and Documents involves the issue of whether or not the image is used as a "source" and attached to a source citation in the Details Page of an individual. For example, if I have a copy of a birth certificate, it may be a photograph but it is also a supporting source document for a source citation on the Detail Page. It appears to me, that FamilySearch intended that the source documents be segregated into the Document section. A reasonable extension of this definition would require that every document that is uploaded into the Documents Section be attached as a source on an individual's Details Page.
The third category, Stories, it is rather easily distinguished from the other two categories. The story is a text document. That means that the information put in the Stories Section of the Memories is in the format of a text file. The text can either be entered directly into the FamilySearch Memories Stories section or can be copied and pasted in from another document. I suggest that the story be written in a word processing document and copied into the Stories Section. This enables the person submitting the story to use the spell checker and other features of a word processing program.
By the way, the program does allow an image that was categorized as a photograph to be re-designated as a document and an image that was designated as a document to be re-designated as a photograph.
All documents, stories and photos should be tagged to the appropriate individuals that appear in the images or in the text files. For more information on the subject please see the "Get Help" section from the link in the upper right-hand corner of each page of the website. For more complete instructions on how to change a photo to a document or document to a photo see the following help Center document:
Changing a photo to a document or a document to a photo
I would also strongly suggest reviewing the Family Tree Training Lessons and Videos in the Learning Center of the FamilySearch.org website. You may not be aware that there are both video and written lessons that explain in great detail all of the features of the FamilySearch.org Family Tree program.