Palliser:
According to the manufacturer, this specific display model can handle 24-bmp.
No, it can't. From the link you gave (!):
Resolution 320x240
LCD color 65k
So it is 16bit per pixel, not 24. This will probably be 5 bits red, 6 bits green, b bits blue. But instead of guessing, lets look at the datasheet (linked from the link you gave)
Display Colors (Color Mode)
- Full Color: 262K, RGB=(666) max., Idle Mode Off
- Color Reduce: 8-color, RGB=(111), Idle Mode On
Programmable Pixel Color Format (Color Depth) for Various Display Data input Format- 12-bit/pixel: RGB=(444) using the 1.38M -bit frame memory
- 16-bit/pixel: RGB=(565) using the 1.38M -bit frame memory
- 18-bit/pixel: RGB=(666) using the 1.38M-bit frame memory
Actually that is a little bit better - it can do 18-bit truecolor. This avoids the problems of greyscale color casts when the number of bits per component is not identical between the three channels. So, starting from a 24bit (8 bits per component) truecolor image, for each pixel, take each component from 8 bits to 6 bits to construct an 18-bit value and write that pixel value to the screen.
If the image is larger than the screen dimensions you will need to downsample it, as well.
You should also check out what the datasheet says about gamma correction.