Masking problem

Eventough I’m totally new to all of this I feel so embaressed to ask the following question, but it just won’t make any sense to me.
And if there’s a simple solution I’ll just blame it on me being tired. :huh:

Extremely simple:
I have an image which I want to reveal with a mask, so in layer1 I have my image and in layer2 I have my mask, which is a movieclip (and placed above the image layer).
For 20 frames I use a motion tween to move my mask from ontop of the image to the left of it.
Then I right-click on my mask layer and apply mask.
What happens?
The complete opposite.
Instead of revealing my image, the mask makes it disappear.

As I scratched my head wondering “what the h#=!?”, I searched for some simple tutorials which told me to do exactly what I’ve done.
I’m totally new to this, but I’ve done far more complex things than this and understood it very well, so I feel both embaressed and dizzy about the whole mather. :cantlook:
Have I missed some option or something somewhere?