Size of generated thumbnails (not optimised)

Hi,

I use plugins to optimise the images on my website. I think the Photoswipe plugin also generates its own thumbnails for the preview and I don’t think they are very well optimised in terms of size/quality. When I run PageSpeed Insights, images are always one of the problems and in particular, I think the ones generated by the plugin.

Is there a way of improving/reducing the size of the thumbnails that are generated?

Thanks