The rules are in the FAQ. "No swearing" is a rule we've had for a long time; in general, a bit of harmless profanity may be overlooked, if it's considered appropriate in the particular circumstance. However, swearing is considered by many to be unpleasant and distasteful; it can easily lead to some serious flaming, which is something else we don't want. There are usually better, more meaningful ways to express yourself than by swearing several times in the one sentence. The more offensive a particular word is, the less likely it'll be that you're permitted to use it. Now, a lot of people do swear frequently in real life, and that's entirely up to them. However, that doesn't automatically mean it should be fine to do so here. The EoFF rules are in place to make an environment that's comfortable and hospitable for the members as a whole. Everyone can be expected to be bound by the rules, whether they like them or not. Afte all, I suspect that a heck of a lot of people here have massive pr0n collections, however I've never seen anyone protest against the "no pr0n" rule. Only the "no swearing" rule gets targetted...
Actually, we've been treating swearing quite lightly in recent months. People have got away with a lot more than they would in the past. However, it's just got worse and worse, with pointless and blatant profanity appearing all over the place.

As for nude paintings... that's a different matter. The "no pornography" rule is strict and will be enforced without mercy. However, each matter is still looked at on a case-by-case basis. If you post a painting in order to discuss its artistic merits, then that'd usually be fine. However, if you posted a nude Venus painting in a thread called "omg hot!!!11!!1" or something (basically, if you're intended to present the picture as pornography rather than art), it'll cause trouble. If you're unsure, then the best approach is to PM a mod or admin with a description of what you want to post and why. They'll tell you if it's acceptable or not.