A professional logo design should always be done as a vector file, which creates images as mathematical algorithms rather than pixels - this allows the image to be 100% scalable to any size without losing any quality. Another benefit of vectors, exclusive to the web, is that images can be created as .SVG files, which do not lose quality when scaled to larger or smaller sizes and are remarkably smaller when loaded on a website (loaded as code instead of typically much larger image files). Vector files are also the preferred file type by most printers, so your logo can be used and scaled to print at any size (Common examples being business cards, letterheads, brochures, display banners, etc..