> Obviously a lot of people contribute to image boards anonymously, but what about people who post content with their handle attached?

They deserve every bit of ridicule that they get for their ignorance.

> Is this a `valid' use of names/tripcodes?

No.

> What about people who sign/watermark their submissions,

Watermarks are gay.

> or post anonymously then take credit later on their own site?

I'm Spartacus!