Thank you!
Yes, it makes sense that my image was way too big for the text to actually see the effect!
It's quite not the obvious tho, also because if I apply the text mask directly on the Image, it shows me the Text's outlines (weirdly stretched I must say..), as seen in picture.
Another strange behaviour is that if I try to scale the font up to the piont that it actually makes the mask visible, it has this strange behaviour. It seems somehow clipped, see picture.
Thank you for the bonus tip about Image Resizing, very useful as well!