Embedded System for a image processing task
You basically have two possibilities:


Go for ARM
Go for Intel (x86/amd64)


It depends how you define your embedded system. There are ARM modules available with four cores, each one running at 1GHz+ (e.g. the Toradex Apalis family is a Multicore ARM Cortex-A9 module but others are available as well).

If you have high requirements regarding RAM and processor speed, probably the better option is to select a Core-i7 or something similar (check out the COMExpress families from e.g. Congatec) and build that into a smaller system.

Now, how do you decide on the right operating system:


Windows Embedded will only run on x86/amd64 architecture; There is no support for ARM processors. This can be considered as a stripped down version of Windows for PCs.
Windows Embedded Compact is available for ARM as well, consider it being the typical Mobile Device platform which is often optimized for reduced power consumption. Also, you are able to add your own drivers easily. So if you want to dive really deep down, this might be your preferred choice.