You have to find the image in your template and it would look something like this:
Code:
<a class="collapse" id="collapse_c_cat{vb:raw forum.forumid}" href="{vb:raw relpath}#top"><img src="{vb:stylevar imgdir_button}/collapse_40b.png" title="Close this box" width="16" height="16" alt="" /></a>
Notice the width="16" and height="16", that is what you need to do for every image that gtmetrix wants you to specify image dimensions for.
--------------- Added [DATE]1347746160[/DATE] at [TIME]1347746160[/TIME] ---------------
Do not resize images using this, gtmetrix wants you to use the correct image dimensions.