Compressing and resizing specific images

Hiya,
I've used WPSmush to decrease the image sizes, and it's somewhat improved my site speed, according to Google:

https://developers.google.com/speed/pagespeed/insights/?url=http%3A%2F%2Fwww.deathtoflowers.co.uk%2F&tab=desktop

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.

Susi