Although these terms are related, they mean different things. The meaning of each is explained below.
Image size: This represents the physical size and resolution of an image measured in pixels. For example, A 10 megapixel (MP) camera may provide settings to take pictures in 10.2 MP (3872 x 2592), 5.6 MP (2896 x 1944), and 2.5 MP (1936 x 1296). A higher image size setting means a larger picture and bigger file size. A lower image size setting makes a smaller picture and smaller file size. Depending on your camera, there may be different sizes available.
File size: This is the amount of space that the image file takes up on your memory card, or internal memory if your camera has one. It is measured in megabytes. The actual file size depends on the image size and image quality settings selected on the camera.
Image quality: This dictates how much compression is used to record the file information. Some digital cameras have a Fine and Standard setting where the Fine setting uses less compression than the Standard setting. Therefore, the detail of a picture taken with the Fine setting is a sharper than one taken with the Standard setting. Similar to the image size setting discussed above, this also affects the file size. Since the Fine setting uses less compression and records more detail, the file size will be larger than one recorded with the standard setting.
NOTE: Some high-end cameras also have a RAW or TIFF setting that provide even better image quality.