How can we improve image markup (width/height/alt text) of Verint OOTB widget images served from the platform?

I believe you can add key/value pairs in the $core_v2_ui.GetResizedImageHtml call of the widget in question - if they're not part of the documentation, I think they get rendered as attributes.
$core_v2_ui.GetResizedImageHtml('img.png', 100, 100, "%{ alt='MyAltText' }")

As this is an 'out of the box' widget, wouldn't this need to be done by Verint so that we aren't the ones to consistently maintain this change every update and potentially break the updates if Verint make any changes to it?

I suppose if you didn't want to touch widgets yes. 

That's one of the problems we have, that we don't want to touch out of the box widgets, as that means we then have to maintain them and make sure they don't break every upgrade. :|