The spawnarg random_skin is a TDM addition. If set, it overrides the "skin" spawnarg of an entity with one random item from the given list. Example
"random_skin" "tdm_fruit_yellowish, tdm_fruit_brownish, '', tdm_fruit_green"
The "," seperates the entries, the spaces are optional. Note the empty entry ('', two single quotes, or empty), this denotes the original skin.
Another example, creating a painting with a random image from these: