Pixel is short for "Picture Element," each of the minute, individual elements of a digital image or display.
Pixels vary in form and function depending on the type of display device.
Dead Pixel by Simon Wicks (CC BY-NC 2.0)
Resolution - the degree of sharpness of a computer-generated image measured by the number of dots per inch in a printed image or the number of pixels across and down on a display screen.
Flowering Spurge by cotini (CC BY-NC-SA)
File Size is the amount of data in an image file, usually measured in "kilobytes" (KB), "megabytes" (MB), etc.
Compression - reduction of a file's size by changing its format.
Sealion by Todd Ryburn (CC BY 2.0)
File Format - standardized means of storing and viewing images on computers. Each format uses a different compression scheme.