Is there any way to track where it comes from, or can they just repeatedly do this at will?
"They" (aka..Them) can do this repeatedly at will. It is extremely difficult to get the actual people doing this. For those people that dont know how this works, ddos stands for 'Distributed Denial of Service'. In practice this means someone is in control of a group of machines, usually just simple hacked home machines like you and I have, and tells all those machines to send an avalanche of traffic to the victim site. Each one by itself can probably not bring a site like wetpixel down, but all together they can easily. Major sites have had trouble with this in the past.
You can sometimes do something about it, but it takes a lot of resources that smaller (in the grand scheme of things) sites dont have. Our company (I work for an ISP) has very...very expensive pieces of hardware that can detect and mostly deflect ddos attacks. This often needs cooperation of other, larger, ISPs, to block this traffic closer to the source. But even that often fails. It just depends on the size of the ddos. If people are dedicated, you're powerless.
The idea that these are kids doing this is a bit of a romantic view of how things used to be. Nowadays a lot of these attacks are done by highly organized criminal groups. You can just pay for it. It's a sad world.
I cant imagine what brings someone to target wetpixel. It's not like this is a very controversial site.
Cor