Better to put them on Plane objects (standard materials). Set the planes to have Backface Cull, turn off "show frozen in grey", then freeze them. You may need to up the Self Illumination as well.
You can improve the quality of the bitmaps by configuring the DX driver.
Max 2016 (SP1/EXT1)
Win7Pro x64 (SP1). i5-3570K @ 4.4GHz, 8Gb Ram, DX11.
nVidia GTX760 (2GB) (Driver 430.86).