I don't know of an image. And you probably already have the settings figured out, but if not, find an image and add the Gert settings to the boot code.
sudo nano /boot/config.txt
For the Gert VGA 666 to work correctly you will need SPI and I2C to be disabled. Verify that by looking in the config.txt for the following two lines:
dtparam=spi=off
dtparam=i2c_arm=off
Then add these lines at the bottom of the file
dtoverlay=vga666
enable_dpi_lcd=1
display_default_lcd=1
You also need to specify your screen resolution. After the lines you added above you will also have to then add one of the following configurations:
#For 1920x1080 60Hz
dpi_group=2
dpi_mode=82
#For 1280x1024 60Hz
dpi_group=2
dpi_mode=35
#For 1024x768 60Hz
dpi_group=2
dpi_mode=16
#For 800x600 60Hz
dpi_group=2
dpi_mode=9
#For 320x240 try these settings
#hdmi_timings=<h_active_pixels> <h_sync_polarity> <h_front_porch> <h_sync_pulse> <h_back_porch> <v_active_lines> <v_sync_polarity> <v_front_porch> <v_sync_pulse> <v_back_porch> <v_sync_offset_a> <v_sync_offset_b> <pixel_rep> <frame_rate> <interlaced> <pixel_freq> <aspect_ratio>
#<h_active_pixels> = horizontal pixels (width)
#<h_sync_polarity> = invert hsync polarity
#<h_front_porch> = horizontal forward padding from DE acitve edge
#<h_sync_pulse> = hsync pulse width in pixel clocks
#<h_back_porch> = vertical back padding from DE active edge
#<v_active_lines> = vertical pixels height (lines)
#<v_sync_polarity> = invert vsync polarity
#<v_front_porch> = vertical forward padding from DE active edge
#<v_sync_pulse> = vsync pulse width in pixel clocks
#<v_back_porch> = vertical back padding from DE active edge
#<v_sync_offset_a> = leave at zero
#<v_sync_offset_b> = leave at zero
#<pixel_rep> = leave at zero
#<frame_rate> = screen refresh rate in Hz
#<interlaced> = leave at zero
#<pixel_freq> = clock frequency (width*height*framerate)
#<aspect_ratio>
dpi_group=2
dpi_mode=87
hdmi_timings=320 1 16 30 34 240 1 2 3 22 0 0 0 60 0 6400000 1
If the resolution that you are looking for is not among these then you can
check this link.I have a CGA running at 320 x 240 (not through Gert666) and I'm not doing much fancy to it. I just made a layout for AttractMode to read the text easier.