Just use a width instruction in the html. I go width="100%" in most instances, and it works fine in Kindle and, from what I'm able to determine, in B&N and Apple etc as well.
Bearing in mind the old Kindles' habit of blowing up every image to full screen or the pixel dimensions, whichever is smaller, I am also conscious of the pixel dimensions of small images. Thus for my publisher's colophon I go width="15%" but also restrict the gif to 100x100 pixels. The result is not ideal on Kindle or Kindle 2 or DX, but it's okay.
Bearing in mind the old Kindles' habit of blowing up every image to full screen or the pixel dimensions, whichever is smaller, I am also conscious of the pixel dimensions of small images. Thus for my publisher's colophon I go width="15%" but also restrict the gif to 100x100 pixels. The result is not ideal on Kindle or Kindle 2 or DX, but it's okay.