How do you "add/include" tags to HTML?

I have an idea of a tag for HTML that map help. Don't wanna say it
outloud here until
I research it enough. But how would someone go about "adding" a tag to
HTML?
for example, if I wanted to create a new IMG tag and call it <BIGIMG>
which does
something,

1. where would I start to see if I can even add it
2. how do you actually CODE something into HTML? is it a process of
defining the tag and then
sending it to some HTML autority which makes all browser companies
include it in the next version etc.?

any help will be greatly appreciated! thanks

Moazzam Ahmed
Dallas