Are the two images in separate paragraphs (lines)? Or just
separated by a space (or no space)?
If they are not in separate paragraphs, I think that the web
preview might wrap them differently from the printed output. Try
putting the top image in its own paragraph with a style that has no
spacing after it (and, of course, the bottom one in a paragraph
with a style with no spacing before it).
If that doesn't work, you could try putting them in table
cells, but you'll have to do some tinkering to get them to snuggle
up to one another.
HTH,
Elisa