ImageField and Page Speed (Questions about query string on image src attributes)

Page Speed ​​indicates to enable proxy caching for these resources, remove query strings from references to static resources.

I'm guessing you mean a reverse-proxy cache here (but the same probably applies to a proxy cache). Any software you use will almost certainly be able to handle the query strings being present. I know personally I've used Akamai, Limelight, Varnish, and nginx all with the query string on the end of the URLs and things work fine. In fact, the query string is actually quite important when using a reverse-proxy cache as it makes it so that if the image changes but has the same file name the cache will be correctly busted and then show the new image. Say you delete the image, and then upload another image with the exact same name, the query string attached to the image changes so the new image is shown.

So... in all. You shouldn't ever really need to turn this feature off, but if you want to you can override the theme_imagefield_item() theme function in your template.php file in your theme.

EDIT: Fixed typos.