Irrlicht Engine

can i have a 2d image placed on screen and make it act as if it were a button when i click it?

check examples.?

Or calculate mouse AND box pos, in event recevier when click check is mous position in button bounds...

Yes you can either with the draw2DImage() function sets or by using billboards, or even 3d models! Often in a game a 'pseudo gui' is much more appropriate as you might want big bouncing jellybean like animated buttons or an interface built out of animated creatures or something.

Using a game GUI should really be a part of the game and be fun to use. If i'm playing a game I want my OK button to have eyes and a nose and to squeak when I click on it.

You can also do it the other way round - give the button an image. Using setDrawBorder(false) and setImage(yourImage) you will have a clickable image with full button functionality.