Parchment doesn't support graphics.
I would try Quixe instead--see ยง25.11. A playable web page in the manual to "release along with an interpreter." You can try it on your own computer to see if it displays the pictures (the game doesn't have to be online--you can open it offline in a browser) and then if the pictures work, you could ask the comp organizer whether it's possible to set up Quixe instead of Parchment for online play of your game.
There's a sample game here http://eblong.com/zarf/glulx/quixe/ called "Sensory Jam" that shows graphics in Quixe, so it seems like it ought to be possible to display pictures. But the version of Quixe that comes with Inform isn't the latest, I don't think, so if that one doesn't support graphics you could try a more recent version of Quixe.