Issue 66: don't sprite transient images
Project Member Reported by stevesou...@gmail.com, Sep 22, 2009
In some cases, web sites are using bg images that change constantly: video
thumbnails, hot products, social contacts. These could technically be
sprited, but logistically would require a new sprite be created every
minute or every page view. 

One approach to avoiding recommending sprites like this is to look at the
last-modified date of the image. This isn't perfect, but it could be used
as a guide.