ip11166 on "[Theme: Flatter]"

Google PageSpeed test complains that Flatter uses the same resolution image selected as a featured image both for full-size display on the page view and for showing as thumbnails previews on a home page when abstracts f4rom several pages are shown. In both cases the full size image is used, although for home page it would be correct to use auto generated by WP thumbnail version of the uploaded image. Can this issue be fixed? Or is there any workaround for this to make sure that either lower resolution files are used for showing pages abstracts preview of a homepage or at least in the HTML code the lower image sizes are shown in the images' attributes.
Thank you!