I inspect lightbox images and found no scrset on thumbnail and big image.Does Lightbox support responsive images?
Hi there 
Thank you for reaching out about lightbox supporting responsive images or not. I’m happy to help! 
Yes, it does support responsive images. Webflow automatically creates responsive variants for all inline images. Check out our help articles on lightbox and how responsive images are handles in Webflow.
I hope that helps. Please let me know if you need further assistance.
All the best,
Wadood