I've done this on the PE-HMI using ARGB 32 BPP using the GLCD to alpha blend in layer 2.
Basically, you have to define the frame buffer for layer2 (layer 1 is the default layer that GUIX uses) and then change the alpha channel to blend it in.
The compositing of the image is described in the hardware user manual and the GLCD driver is documented in the SSP Software manual.
-Gary