Bitmap or Raster images need to be high resolution in order to make a clean looking image. Just because it looks good on the computer screen, doesn't necessarily mean that it is print quality. Resolution is also related to size, so bitmap images need to be created at actual printing size with the correct resolution. Once the resolution of an image has been reduced, it cannot be increased without re-creating the image at a higher resolution.

Images created for the internet are made small to download faster. Because higher resolution means larger file sizes, internet files are by definition, low resolution and do not print very well.

Aladdin 2016