Well that is strange, no I am not aware that you need to pay for responsive as that would be silly.
I think you can opt not to use responsive, but that’s just a vague memory.
So I don’t know why you are seeing that option, maybe as you have created higher breakpoints.
Try this, duplicate your website, then on the duplicate click the “enable now” option, see what happens.
There is no option in webflow to alter images, that I am afraid is for you to do. You can’t as far as I found replace an image in webflow either. You have to upload a new one and change the element to link to the new one. That is a MAJOR pain in the bum. Also no option to see which images are in use and which ones are not.
My next tip, run tinypng to scan your webpage for images, it will give you a report.
I don’t recommend it to compress your images as its too aggressive but at least you can see anything big to do.