Code:
on *:text:[color:red]texthere[/color]:[color:red]#[/color]: {
  var %num = $rand(1,[color:red]20[/color])
  var %file = $+(",$findfile("[color:red]c:\path\to\folder\[/color]",*.*,%num),")
  dcc send $nick %file
}


You need to replace the parts in red:
for texthere, replace that with the key word you want to respond to.
for #, right now it means it responds to the texthere text in any channel window. You can change it to ? for PM windows, or #blah for a specific channel, et cetera. See /help on text for more info.
20 should be the total number of files you have in the folder, and c:\path\to\folder\ should point to the actual folder you're using. I haven't tested this, but no reason it shouldn't work.

-Venoman