A few things needs to be clarified.
If you want a direct process of downloading to gallery, there should be two solutions for ios.
1. Change a few lines of code to existing cordova-file-transfer ios oc code. Just add few lines with:
UIImageWriteToSavedPhotosAlbum(image,self,callback,nil)
2. Use an image tag to receive the image, then use a famous plugin Canvas2ImagePlugin to convert the image using canvas directly from js. This also goes directly to gallery.
For android, it is okay just saving the image to sd-card or system image folder directly.