Just a note to myself so I remember how to do it. Actually, as long as you stick to pdftex, it’s pretty easy:

\\input graphicx

Hello.  here is some text!

\\includegraphics[width=3in]{/tmp/foo.jpg}

And here is some more!

\\end

You have to give the images a width, so that it can figure out what it wants to do in terms of dpi. There is no inherent dots-per-inch size to a bitmap file… just a size in pixels.