There are other more powerful alternative comparing to ResberryPi at almost the same price
They are called HDMI Android Dongles running the latest Android ICS 4.0.3 with ARM Cortex A9 1Ghz CPU, Mali 400MP Graphics Processing Unit OpenGL ES v2.0, they are able to play almost any video format with decoding capability up to 1080p,...BUT we need 3.2 released in order to take advantage of Mali GPU. However if you would simply looking to display JPEG alone this product is more than enough.
We have this products in-house and have been playing with several signage player and I hate to say but Media Signage experience is not the best so we are really looking forward to 3.2 release.
There is another product coming out very soon, similar to the one above but with HDMI pass-through feature. This product contain an HDMI-IN and HDMI-OUT meaning that you can add content on the top of Live-TV. I am not sure if MediaSignage R&D is willing to take advantage of such innovative products.