Dithering: Dithering is a technique used by GIF (Graphics Interchange Format) images to reduce uneven edges at low resolution. This will decrease the file size.
Link for pictures to compare how the file size decreases.
Anti-aliasing: In game’s there are sometimes jagged edges. Anti-aliasing detects in this by scanning for them and evens them out
Increase resolution (re-sampling): Resampling affects the file size of an image. Information can also be removed from the image (remove the pixels) which is called downsample.