To allow users to quickly see a second image when hovering over your products in the list layout or widget embed, simply enable the Image Swap on Hover switch.
For hosted stores, this can be found in Sales Channels -> Hosted Store and then in the Includes menu in the left-hand sidebar, look for the Image Swap on Hover switch. Enable it, then save your store and the change will be reflected immediately
For embedded stores, this can be found in Sales Channels -> Existing Website and then in the Includes menu in the left-hand sidebar, look for the Image Swap on Hover switch. Once enabled, you'll need to copy the newly generated embed code again and paste it where you'd like to display your products.
Please note, options are specific to each embed code, so if you have multiple embed codes throughout your site, you'll need to replace each one you'd like the changes to apply to