!important is “dangerous” (OK, maybe too strong a word) because you can’t override it. On gokeeffe’s site, there is (or was) a rule declaring that an image can never have a border, anywhere on his site, under any circumstances. That’s the effect of the !important. It’s far too absolute a statement, as the issue above shows. If there are some images in some circumstances that should not have a border, it’s better to target them specifically—either by a special class, or even just with descendant selectors. E.g. if you want to make sure no images in a sidebar even have a border, use something like #sidebar img {border: none;}
It’s fine to ask questions. That’s what a forum is for. I’ve learned a lot here that helped me when I came to similar situations, so yay for preemptive learning!