$html=<DATA>;
$html =~ s/<IMG \s+ 	           #match the IMG tag
    SRC \s* = \s* "[^"]+" \s*      #match the Source
    (ALT \s* = \s* "([^"]+)" \s*)? #match an optional Alt
    >                              #end of tag
    /'[image' . ($2 ? ": $2" : '') .']'  #print the image stuff
    /sgixe;
print $html;

__DATA__
<IMG SRC="foo"><BR>
bar bar bar<BR>
<IMG SRC="foo" alt="bar">