I don't know if anybody is even interested but as a little follow up I'd like to quickly explain how I was able to solve the problem. I simply saved the image to my Pythonista library and uploaded it from there:
imgSaved = image.save(fileName)
And then upload it using the sftp.put command:
sftp.put(fileName, remoteFilePath + fileName)
os.remove(fileName) the file can finally be removed from the Pythonista library to keep everything clean.