using .gif images for messages

[bbxrider]

(f.lname.value.length < 1) // validate last name length
? showImage("lastnameerror", "fieldempty.gif", true)
: showImage("lastnameerror", "blankimage.gif", false);

in this code, its not mine but available at javascript.internet.com, it uses
..gif images to display error messages instead of alert, which i like, but
have 2 questions
1. how is it the image is being positioned at the right spot, right next to
the input control
2, all the images have to be brought back from the host site? and possibly
over and over again if the user keeps entering incorrect data. could that
not cause transmission delays? etc.
or are the images brought over with the html page when page first
requested?
or are images somehow cached by browser to avoid having to keep going back
to server??