It does not seem like a Rockbox issue. It happens on DX50 as well. Check Cholero's post about it in the DX50 rockbox thread. It appears to be an issue with LCD screen used on both devices.
Keep us posted if your workaround can fix it.
My Idea with 321x241 wallpaper not function. Wallpapers more than 320x240 are not displayed.
I have one other problem. How much size of code in .wps file is maximum? I want give equalizer to my theme, but eq gain parameters in rb are -240 to 0 to +240 - it is 481 steps. I must have 10x:
%xl(Q,eq.bmp,0,0,25)
%V(151,104,16,49,-)
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When I have 8 bands, all is OK. But when I give to code 9 bands, theme not function. But I need 10 bands
I try it over Bar function:
%xl(R,eqbar.bmp,0,0)
%V(151,104,16,49,-)
%St(0,0,15,49,R,vertical,eq band 0 gain)
But Bar is displayed full without any effect of gain setting.