I found a bmp image file online, right clicked on it, saved it as bmp format in a local directory. Using MS Paint, I could open and view the file. Paint says the picture-file is 42.2K bytes, 240 dpi, height 0.5in, and width 0.5in. But when I tried to insert the image into a plain TeX-coded document using XeTeXpicture, there was just a blank space where the picture should be. I bracketed the bmp image with letters a and b just to set the context. From an earlier question which was answered by Akira Kakuto, see Using Xetex to Include bmp file into document using Plain TeX I was already given a sample bmp file that worked. So it must be something about how I am saving the bmp image from the internet.
Microsoft Internet Explorer (and Edge) will allow users to chose save format for a web image. Given the choice most Windows users would prefer to save as 24 bit BMP (16 million colour) rather than 8 bit (256 colour) GIF
Actually in this example its 32 colour greyscale as BMP or 18 colour as GIF so there is a noticable difference.
Microsoft do not set a DPI recognisable by some graphics apps in either of these downloaded formats (although MSPaint will often show 96 dpi as its working default)
Thus in order to ensure TeX applications can calculate a size for subsequent placement we need to RE-save as a convenient format such as 96 dpi PNG (32 bits)
Oddly if you then resave the new PNG as 24 bit BMP it magically is redefined as a 96 dpi BMP !!
Using this example
% xetex test.tex
this is raw downloaded bmp = invisible
\XeTeXpicfile raw-seal_3.bmp width 6cm
\XeTeXpicfile nh-seal_3.png width 6cm
modified bmp visible
\XeTeXpicfile mod-seal_3.bmp width 6cm