After the ICS Update Xperia Ray and other xperia models usage are changed alot. Same is the case on the way to take the screenshot on xperia ray and other xperia phones after ICS update.
So it is the way:
Press these two keys one after another within a second to take the screenshot from you xperia ray or other xperia phones with ICS :
POWER BUTTON then VOLUME DOWN.
It might be difficult at first but after using it for some days it will be easy for you. Here are some of the screenshots which I taken from my Xperia Ray with the ICS update: