I ask for credit on my icons because I actually do a lot to the original image. If I was just cropping and resizing, credit wouldn't be a big deal because anyone can do that in a couple of seconds. But I correct color, saturation, brightness, etc. to make the image look better. I also take the time to scour the net for pictures or, in some cases, make my own caps of shows/films. AND I make manips, which takes a lot of time.
Not only that, but I know when I see an icon I like, I want to know who made it so that I can use it and credit, too. I always, always credit if possible and I think it should be a common courtesy. But that said, when I see someone using my icons and they don't credit... I drop a polite note and they almost always add the credit. Most people just overlook it.
As for coding, unless it's something incredibly complex or specific, then it's nothing you can't figure out by googling HTML codes. If I see coding I like somewhere, I almost always go and handcode it myself from the floor up to recreate it. Everyone's style of coding is different - just like how people write differently - so I like to put my own stamp on coding. I only credit when I'm copy/pasting something directly because, otherwise, it's all written by me.