As far as “transferring” using CSV, APIs, etc. we just did this from a WordPress blog. We had a lot of images that needed to be manually reuploaded as we hosted our images on the servers that hosted our WordPress blog.
So the whole thing ended up being really manual and hard (a lot of copy and paste). But we got it done. Fortunately only about 40 posts. If you had more than this, it would be a nightmare. Not sure if there’s a slick way to upload images to Webflow’s CDN using an API. Otherwise, it might be best to just use a third party CDN for images in the future so the blog can be migrated easily.