I had been munging on a regex as well (just an exercise), and I think the extended regex clearifies a bit:

$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">
[download]

This works, but keep in mind that the IMG tag is still valid if for example, the SRC and the ALT are reversed in order.

That's why HTML::Tokeparser (as Desdinova pointed out already) or maybe even (if the HTML is yours) Template Toolkit are better approaches.

Cheers,

Jeroen
"We are not alone"(FZ)