Use a transparent bitmap. This feature is only available with high-color firmware versions; the “vers” command will show the version you are using.
To use transparency, create a 24-bit color bitmap with a fixed color for the transparent (non-displayed) pixels. Let’s say you used pure red. In 24-bit terms, this color is hex FF0000 (RRGGBB format). The high color firmware uses RGB565, that is, 5 bits of red, 6 of green, and 5 of blue. So it would be best if you converted the 24-bit color value to 16-bit. In this example, it would be hex F800. Then add the string “.trf800” to the name of the bitmap, and it becomes a transparent bitmap. (The transparency value has to be put in the filename because the .BMP format cannot specify a transparent color value). To get the 565RGB value corresponding to a 24-bit RRGGBB color, use the ‘S’ command to set the fg/bg color in RRGGBB format; the command will respond and show the RRGGBB values and their corresponding RGB565 values.
See the manual section “Working With Bitmaps” for more detail. You can download the manual by locating your module in the Touchscreen Display Module menu and then the Documents tab.