The copyright status of many of these is dubious at best, you should expect to get sued if you try to make money off any of them.

For sheer visual impact, my favorite two are lush.jpeg, and oldmill.jpeg.

Particularly historical are uscgirl.gif and baboon.jpeg (actually a mandrill), at least one of which seem to have appeared in every image-processing paper of the era.

More modern images on net:
Cindy Crawford, a fan's page.
Cindy Crawford another fan's page.
Playboy official homepage.

123814 000dg.jpeg
102798 1990_zr1_corvette.jpeg
74563 FijiChief.jpeg
156371 Monument-Valley.gif
271163 a_day_in_the_park.jpeg
17408 albert.gif
9756 anulka.jpeg
66132 apr89.jpeg
231795 ariz_falls.jpeg
209240 ariz_falls2.jpeg
108600 baboon.jpeg
15198 balloons.jpeg
119590 batman3.jpeg
12876 blast.jpeg
95872 blue-angels_f18.gif
26818 bud1.jpeg
17520 corysia.gif
22974 countach.jpeg
62620 cowgirl.jpeg
11900 danielle.jpeg
14080 debbie.gif
16384 debee3.gif
12210 debrajofondren.jpeg
12516 denisemcconnell.jpeg
39360 dietcoke.jpeg
17199 dragon.gif
15480 dumbell.jpeg
11099 eagle.gif
15872 earth.jpeg
24891 earth1.gif
17978 earth2.gif
17704 earthrise.gif
6343 earthrise2.gif
29696 earthv.gif
70549 edwinaldrin.gif
23451 eighthea.gif
86016 electric.gif
6656 elf.gif
200115 elle.gif
6631 emperor.luke.gif
11264 enterprise.gif
12855 escher3.gif
77606 euro01e.gif
20017 euro01v.jpeg
4948 eve4.gif
18687 eye.gif
3072 eyes.gif
44825 ferrari.jpeg
6912 gifdecode.gif
8320 gifencode.gif
9216 giftest1.gif
51110 gillian.jpeg
32975 girl.gif
16443 girl1.gif
16893 girl2.gif
20506 girl3.gif
20084 girl4.gif
20635 girl5.gif
17988 girl6.gif
15210 girl7.jpeg
6272 girlorb.gif
17656 girlsplash.gif
9605 gulls.jpeg
17271 gwtw.jpeg
27598 horse.jpeg
89526 hubble.gif
103081 iceworld.gif
26602 juggler.gif
51879 jul89.jpeg
7283 julia.gif
15880 jupiter.jpeg
18107 jupiter3.jpeg
57417 king-tut.jpeg
144677 kitty.gif
68701 krissa.gif
11843 kristine.jpeg
13087 kymberly.paige.jpeg
10978 lisawelch.jpeg
129196 lunar.gif
382284 lush.jpeg
24841 m33.jpeg
24043 madonna.jpeg
18233 mariel.jpeg
24599 mash.jpeg
10477 mike.jpeg
12502 mikki.jpeg
10328 mikki2.jpeg
26452 monkey.gif
26963 moonflag.gif
7808 mtvlogo.gif
19163 mz.gif
183946 newton-basin.gif
42441 nm.jpeg
50671 oct89.jpeg
419357 oldmill.jpeg
20263 orionc.jpeg
13087 paige.jpeg
70528 paulina.gif
48186 reagan.jpeg
32768 roger.gif
27031 ropes.jpeg
52835 sanfrancisco.jpeg
51622 sigirl4.jpeg
57967 sigirl5.jpeg
13281 sondra.jpeg
39699 sr71a.jpeg
32133 tiffany.jpeg
22528 tifiny.gif
13090 tiger.gif
16896 tiger2.gif
63889 tube.jpeg
30720 tutankamen.gif
14910 uscgirl.gif
8192 worldmap.gif
21429 woz.gif
52450 xlast.jpeg
40875 zebress.gif
15713 zoe.jpeg