The thumbnail images of a lightbox takes adavantage of the srcset img-attribute. That is fine.
But the big image of the lightbox always seems to show the biggest image uploaded as there is no srcset attribut available. This is a bit troublesome, because if you use the lightbox on a mobile device you will get the heaviest (filesize wise) image.
Could you please confirm this behaviour, and if yes, do you plan to include the srcset attribute as well to the big image in the lightbox?
That is correct, Webflow lightbox does not use responsive images at the moment. You might want to try the Wishlist to recommend this excellent idea!
Please search the Wishlist for upcoming features, and if there isn’t one for your requested feature yet, you can create a wishlist request.
You can also subscribe to wishlist items to stay updated to announcements regarding the wishlist item.
Wishlist suggestions and discussions are off-topic for this forum, as we have a dedicated site for that. You can also ask such questions during occasional Webflow live Workshops where questions are taken from the users, and updates are provided on upcoming features.
If you do not like this policy, feel free to bring it up on Meta.
If you do not like the wishlist, feel free to provide Feedback.
For possible workarounds (if any), please use the Search feature on the forums.