I believe (and could be wrong as I dont ever recall having an image of mine in that section..LOL)  That it is based on HOW many times an image is rated that day rather then what type of rating they get.. So in theroy you could have an image that is 6 months old but if 40 people rated it today (even if they all rated it with 1 start) it would, or should end up in Todays top rated, could be wrong though

Renderosity's "problem Child"
Support Hydrocephalus research.. because a Shunt is NOT a cure!