The simplest way is to set every odd pixel in the image to the mask color:
XoXoXoXoX
oXoXoXoXo
XoXoXoXoX
oXoXoXoXo
which unless you look very close gives the effect of the image being 50% transparant.
XoXoXoXoX
oXoXoXoXo
XoXoXoXoX
oXoXoXoXo
which unless you look very close gives the effect of the image being 50% transparant.