That's a good tip, thanks Nupsi, that one was part of the solution but in the end I had to set NULL the ONOUT parameter when ONCLICK because it seems it's the one interfering here. I thought that ONHOVER (or ONOVER) was the one sending the command to tween alpha up but after a few tries erasing this instruction seems that ONOUT is the culprit here. I thought ONOUT worked when mouse pointer left the image but now I'm confused, because it's activating even when the mouse haven't left this area.
By the way Hans, it is possible to set (or tween) alpha to a value greater than 1, it doesn't make any sense for an opaque image but for the ones with semi-transparent pixels it forces them to reduce it's alpha. For instance, if you have a logo with a white glow around it, it will glow more, because the "glowing pixels" will turn more opaque. Try it!
Best regards
Luis