Well, yes and no.
At first I copied and pasted it into a graphic editor (it's on another Pc, I don't remember its name right now) were I was able to reduce the color depth, change the constant, and save it. Afterwards I realized it was 3 pixels wider and longer than needed, so I thought to cut and re-save it within the same editor but then there was no way to use less than 8 bits/pixel.
At last I used an icon editor where you can freely set the icon's dimensions. That did the trick.