DISCLAIMER: I am not an attorney!
Technically, in most jurisdictions "pornography" has to involve aroused genitalia (hard-ons and the like) and/or intimate contact with genitalia. It's not always clear whether a particular image is pornographic, but if it involves genitals and feet, chances are it will be considered porn. Images of kissing or licking feet with no genitalia involved probably wouldn't be considered pornography in the legal sense.