That will not be easy at all. FTP servers works on scripts and you will have to design your own script interpreter to make it work.
The other issue is that you need to be able to decide what the URL your image was saved under so that you can call it from Thunkable.
You will need API to work with it.
I would recommend using Google sheet. Thunkable can connect directly to sheets and you can save images in sheets. I have never tried it but worth trying.
Thunkable uses Cloudinary, Is there a reason not to use it?