Gotcha, and yes, it can provide some benefit. However, if the WebP version is much smaller than the original, it may still be worth it to use that, especially if you have a lot of mobile users. The smaller image may save more time than the delay from loading.
As I mentioned previously, it’s really based around how the image is being served. Because these are in CSS files defined as background images, they can’t be touched, except by lazy load. Well, technically you could use our Easy IO CDN and be able to do what you would like, but that’s a paid option.
Really the only option is to use lazy load and get WebP or not use either. Even without WebP, it may still be better, considering how much the WebP saves. I guess if you really wanted, you could hard-code the WebP as the background image. You would lose the fallback ability but you wouldn’t have to lazy load it.