Got no idea how to do it.
But if it's for an action game, much of the CPU time would be spent to check each of the many edges of such irregularity shape!
That would only work w/ good performance if we had GPUs (or APUs) w/ hardware perfect sprite collision feature like the computers and videogames of old.

Well i still hope someone knows how to do this, because in some kind of interface whit non circular or square buttons this will cause problems.

Traditional way is to have a mask, ie. just the silhouette of your sprite.
You draw the mask in a PGraphics, and check the mouse position against this PGraphics: if the pixel under the mouse position has the same color than the mask, there is a hit. Otherwise (color of background), it is a miss.
Costly method is to have a PGraphics of same size than the sketch area, and do a real test. Smaller method just has a PGraphics as the same size than the bounds of the mask, and applies an offset to the mouse position to do the hit test.

Llike PhiLho said, the easiest way is to mave a mask. You can creat a boolean matrix (rectangle matrix) ans set every 'pixel' of that matrix true if it was a part of the shape of the image.

It would be cool if this worked, but i have to admit i never worked whit pixels yet.... could anyone link to some kind of tutorial of some kind?

It's really simple: