Hey @veganedge! The tool is useful when coding your own site, but can present misleading results for sites built on a CMS platform, like Squarespace. You can find some information on this here: https://support.squarespace.com/hc/en-us/articles/206545657#toc-google-s-pagespeed-insights-errors There is also a great guide on formatting images for web here, if you want to check it out: https://support.squarespace.com/hc/en-us/search?commit=Search&query=Formatting+your+images+for+display+on+the+web&utf8=✓
Hope this helps!