[TxMt] Snippet help: get image size?

With a selection of "x.jpg", I'd like a snippet for the html context that can insert

<img src="x.jpg" alt=" x.jpg" width="164" height="234" border="0" class="box1">

I have a snippet that works:

<img src="${1:${TM_SELECTED_TEXT:TM_CURRENT_WORD}}" width="$2" height="$3" border="${4:0}" class="$5">

but it only uses default values for width and height. Are there shell options to get these?

Thanks for the help, tim

PS: if you find yourself making lists out of lines of text, this helps me

<${1:ol}> ${TM_SELECTED_TEXT/(^.+$)/\t<li>$1</li>/g} </$1>

The secret of success is constancy to purpose. Benjamin Disraeli