@mfs12 I keep looking for what may be wrong. I hope I succeed too so that it will help those who could face the same problem in the future 🙂
The image dimensions are good. Here is the same image displayed as a button, with the AddIconButton() function and an uint8_t array of my image generated with the bmp2c tool thanks to your first message:
The button is large, but the image dimensions are the same than those displayed with the StaticImageField() function.
Anyway, thanks for your help, that was very kind! 🙂