Webflow does not include an image manipulation tool or functions you can access directly. There is automatic resizing of your upload, creating versions of the file that are optimized for different viewports down from desktop. This is explained in greater detail here:
A quick search of the forum turned up this thread you may consider reading.
I crop, resize, and optimize on my desktop before uploading to Webflow, In order to have complete control of images on sites I build. As a professional photographer also, image quality, and fast load times are very important to me, so I don’t mind the extra work to get the best results.
Personally my favorite tool for that is Graphic converter on a Mac.
It’s batch processing is far superior than anything else I’ve ever seen and used.
When clients want to process their own images, I have a web app that stores the site requirements and creates custom sized and cropped images for them. I use a processwire CMF built engine for that. Hope this helps. Good day.