In Snd, display the sonogram you want.
-- zoom and move the time window to see the desired audio segment in time domain
-- hit the `f' button to display frequency domain
-- if you only see a 2D FFT of one instant you need to choose Options:Transform Options:sonogram
-- tune up the sonogram, e.g., choose 1024 points, dB amplitude
-- (to save tunings for next time, choose Options:Save Options)

Now that it looks ok,
-- choose File:Print
-- go with graphics output to "snd.eps" in the current directory or change path if needed
-- hit print, then dismiss

Use gimp to edit and reformat the graphics file.
-- in the directory with your file, type "gimp <filename> &"
-- first time using gimp, just accept the default configurations as it installs itself
-- accept all defaults as it opens the .eps file
-- right-click on your graphics display window to find the operations you need
-- e.g., crop to just what you want in the final product
-- save result as type .png (or .jpeg) for web use, default compression leve is ok