The images that are used for Ranks have no alt attribute for use with assistive technologies.
Sample Source code: <img width="35" height="35" src="http://nightly2.sakaiproject.org:8082/access/content/attachment/4a2c62c8-4063-43f9-ba07-0a1f8f39bee0/piq_151281_400x400.png"/>

I would think a good alt attribute would be something like:
[RankName] image

This is needed for compliance with the following standard
Standard
WCAG 2
Level
Level A
Category
Success Criteria 1.1.1

All images (non-text content) must have a meaningful text alternative that serves the equivalent purpose of the image.

Text is considered accessible to almost all users since it is handled by screen readers, non-visual browsers, and braille readers.

The content of the alt text will vary based on the purpose of the image:

Active images: For active images (images in inside anchor a tags), the alternative text should convey the purpose or function of the link. Do not include "link to" in the alt text since screen readers will announce that the image is a "link graphic" before announcing the alt text.
Non-active images: If the image is not active, but conveys information, then the alternative text must convey the same information.
Decorative images: If an image conveys no information, or is redundant, specify that with null alt text: alt="" (no space between the quotes)
Images of text: If an image is an image of text, the alt text should be the same as the text in the image.

If a short description can not serve the same purpose and present the same information as the non-text content (e.g., a chart or complicated diagram), provide a long description using the longdesc attribute. See Example 3 below.