An open wifi network is not excuse enough to explain why you're computer is infested with child porn.
If someone else did it through your network, your computer wouldn't be the one with the dodgy pictures, theirs would.
Mostly they wouldn't store it on your computer to incriminate you, because you would more than likely notice, and you would more than likely cut off their access. I suppose it is possible, but it doesn't make a lot of sense. It's not a particularly good way of convincing a jury there's reasonable doubt to your guilt.
Now if you're claiming they used your wifi connection to download porn, and your computer as a spambot to distribute it, then you would presumably have some sort of evidence to back that up.
Like buying an antivirus program, scanning your computer and printing out the logs.
No? Well you're stuffed then.
And of course, as someone else has already pointed out - it's a bit of stretch to say you conveniently had a writable CD in your burner and they conveniently decided to make a copy for you, without your knowledge - then after thinking about it, snuck in your house and hid it.
Sounds like the jury and the cops did their job and a twisted bit of work is safely locked up - where he'll enjoy a new kind of pornography with himself playing the starring role.