Indeed @Roey, it is not possible to update a WF CMS item with zapier atm.
I didn't create any zap like this yet, but maybe you can try to create a new collection for your uploadcared images, and a reference to this collection in your "Gsheet" collection.
Make a zap that creates an item in your Gsheet collection (with your form data). For the uploadcare image, make another zap that creates an item in the new collection.
You will have to select the reference manually each time you create a new item using this procedure...
Maybe there is a better way to do that, I'd be happy to know
Hope this helps