Failed to select source?

What is the cause of these types of errors?

2000/02/23 20:27:02| Failed to select source for 'http://www.mujweb.cz/web/rarex/variety/pkbutton.jpg'
2000/02/23 20:27:02| always_direct = -1
2000/02/23 20:27:02| never_direct = 1
2000/02/23 20:27:02| timedout = 0
2000/02/23 20:27:02| Failed to select source for 'http://www.mujweb.cz/web/rarex/variety/sexillustratedbig5.gif'
2000/02/23 20:27:02| always_direct = -1
2000/02/23 20:27:02| never_direct = 1
2000/02/23 20:27:02| timedout = 0
2000/02/23 20:27:03| Failed to select source for 'http://images.netradio.com/website/redirect/r1.gif'
2000/02/23 20:27:03| always_direct = -1
2000/02/23 20:27:03| never_direct = 1
2000/02/23 20:27:03| timedout = 0
2000/02/23 20:27:04| Failed to select source for 'http://images.netradio.com/website/redirect/r2.gif'
2000/02/23 20:27:04| always_direct = -1
2000/02/23 20:27:04| never_direct = 1
2000/02/23 20:27:04| timedout = 0

-----------------------------------------------------
Brian Feeny (BF304) signal@shreve.net
318-222-2638 x 109 http://www.shreve.net/~signal
Network Administrator ShreveNet Inc. (ASN 11881)
Received on Wed Feb 23 2000 - 19:36:22 MST