Meaning of dvipdfm warning with jpg and png images?

[Gerry Wolff]

In a book that I am writing using Miktex, many of the images are EPS files
and these cause no problem.

But for some of my figures, I have found it necessary to use JPG or PNG
files like this:

\includegraphics[width=0.25\textwidth, bb=0 0 324
500]{00_Frontmatter/photo2.jpg}

When this is processed as follows:

latex %1
bibtex %1
makeindex %1
latex %1
latex %1
dvipdfm -p letter %1.dvi

I always get a warning message from dvipdfm for each JPG or PNG file like
this:

"Warning: Ignoring user specified bounding box for raster image."

The warning is puzzling because I am currently getting the right result and,
if I take out "bb=0 0 324 500", I get the wrong result.

Can anyone explain what is going on and, perhaps, what I should be doing
differently?

With thanks,

Gerry