As for image manipulation, sadly, the Pi is fairly fast by embedded standards, but running general-purpose software on a full-blown OS takes a lot away.
For comparison, the original Xbox used an x86 of the same clock speed.
Which is why we any predefined image manipulation should avoid the ARM core and jump straight to the rather powerful GPU. I think it even supports OpenCL, so there should be software out there that could work with it.
>For comparison, the original Xbox used an x86 of the same clock speed