The app allows to choose between lossy or lossless compression to reduce the image size:
With lossless compression, a store owner doesn't have to sacrifice any bit of the original quality of the image. However, sometimes it is more reasonable to give away a small amount of the image quality (most often invisible to the human eye) to significantly reduce the weight of website files.
The app uses both these methods to best fit the image and site. Dropshippers may choose whichever works best for them.