I've used WPSmush to decrease the image sizes, and it's somewhat improved my site speed, according to Google:
But it's still highlighting a handful of images that could be decreased. I've had a look at them in the library, and it looks like they have already been smushed, and I can't see a way of doing that any further. Am I missing something – is there a way of losslessly compressing them, or will these files always be flagged by Google?
I should say, I'm no expert in WP, so apologies if this is a daft question.