armStoneA9r2 comes with LVDS and HDMI, other boards have RGB-LCD as well. In this case only the LVDS port is active.
To display an image via HDMI, you need to activate the HDMI port.
This can be done in the Device-Tree. Specific file: arch/arm/boot/dts/armstonea9r2q.dts in Linux source code.
There you can find the line
#define CONFIG_ARMSTONEA9R2_MXCFB0 DISPLAY_LVDS
This needs to be altered to HDMI
define CONFIG_ARMSTONEA9R2_MXCFB0 DISPLAY_HDMI
Now recompile the Device-Tree and install armstonea9r2q.dtb on board.