There are all kinds of file types that are used online. The quality of the graphic and the extension type is going to have a major impact on the kind of results you get. For example, JPEG is a very common file type that is often used due to how much compression it can have, but the quality suffers because of this.
Then you have other file types like GIF, which is often used to create short framed animations. There is also PNG, which is commonly going to have much higher quality while maintaining decent compression levels. Learning about file types is going to be very important for anyone involved in marketing.