It's just a trick to have a transparent picture overlaid over a background for browsers that do not understand the LAYER tag.
Just wrap the foreground picture within a table that uses the background picture as a background.
This will be used in
TWikiDrawApplet to annotate pictures.
Example: (remove the space after the % to have the correct text)
<table border=0 width=... heigth=... background="% ATTACHURL%/drawing.background.gif"><tr><td>
<img src="% ATTACHURL%/drawing.gif">
</td></tr></table>
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AndreaSterbini - 10 Apr 2001