Public Domain Images
Creative Commons Images
Vector based mages (.ai, .eps, .svg, and some .pdf) are based on a mathematical equation. For example, if your image is a circle in vector format, that circle is simply a mathematical formula. If you resize that circle by 1000x, the mathematical formula would still calculate that circle in proportion. This means that no matter how you resize vector images they will scale properly and there will never be any pixelation.
Raster based graphics, (.bmp, .jpg, .png, .gif, .tif, etc.) are based on a grid of pixels or dots. The pixels represent certain colors that make up a bigger image and are resolution dependent. When you resized a raster graphic, they eventually start to pixelate due to the pixels being “stretched” beyond their original size.