It might be helpful for you to first read General Guidance and How to use the Forum and the topic of its first link How to use this forum - please read.
GxEPD2_WiFi_Example.ino is just an example. It may be helpful for you, but you may need to adapt it to your needs.
You didn't report if the example works for you with the bitmaps it shows.
If it shows only black with your BMP file, the reason might be that all pixels are too dark for the threshold for white in the example code, see e.g. line 532:
whitish = with_color ? ((red > 0x80) && (green > 0x80) && (blue > 0x80)) : ((red + green + blue) > 3 * 0x80); // whitish
Nobody can help you with this, as your mirror.bmp is "very local" 
, nobody can access it.
Jean-Marc