It's relevant to note that when it comes to things like Mona Lisa's eyes, or the couple gazing into each other's eyes on the Fountain of Youth picture, you need to click exactly on the item you're specifying; clicking between her eyes won't do, nor will clicking between the lovers.
RickyTheFish
Feel free to delete this comment when you've updated the guide - no sense having the walkthrough twice in one post, right?
1) Click the signature midway between the two people.
2) Click the linen covering the man's groin.
3) Click the dog.
4) Click the swirl of clouds in the upper-right.
5) Click Jesus.
6) Click the cloth the woman is holding up to her eye at the upper-left.
7) Click the object in the baby's hand.
8) Click the eyes - I believe either one will work.
9) Click the shell.
10) Click the sword in the right hand of the left-most person, the book in the same person's left hand, and the flower in the right-most person's right hand.
11) Click the creature in the bottom-middle of the painting (next to the satyr's head) for the optional portion, then click the pan to the left of the satyr's left leg.
12) Click the hourglass.
13) Click the couple in the far lower-right corner behind the bushes.
14) Click the arrow on the ground.
15) Click the person flailing in the water, in the lower-right near the boat. This one tends to be very picky about where you click.
16) Click the person emerging from the cloud in the upper-right.
17) Click the bird in the two boys' hands.
18) Click the dog's collar.
19) Click the mouth of the cave at the upper-left.
20) Click the lute.
21) Click the people in the background (to the right of the woman's head) for the optional portion, then click the mirror at the bottom-center.
22) There are six brownish-gold statues to click on; one at the upper-left, one at the upper-right, two at the lower-left and two at the lower-right. Note that when I say upper-left etc., in this particular case I mean the upper-left of the rectangular central portion, not of the painting as a whole.
23) Click the ass of the figure at bottom-center.
24) Click anywhere in the bottom-central panel.
25) Click either the large golden wine jug or the wine bowl next to it.
26) Click the man's face that is just above the person immediately to Jesus' left.
27) Four things are needed: Jesus is found in the center, the wine-pourer is in the lower-right, the butcher is in the center just above Jesus (he's wearing green), and Suleiman is on the left, dressed in yellow sitting at the table.
28) Click the peacock.
29) Click the person dressed in yellow in the lower-right holding a globe.
30) Click the ferryman at bottom-center immediately to the right of the cross.
Knuckstrike
Thanks, I'm going to finish it now, but I will change some things. Some of your descriptions are clearer.