Don't spawn images if image of this type is already used on the website

[Maingron]

The script shouldn't spawn a test / dummy image to check if a certain image type is supported if an according image is already on the website.

For example: If there is a .png in the footer and it's bigger than **px, this image can be used for testing.

It should be considered that CSS or other things could influence the actual image size.

Implement after #9

Also maybe don't check for image types that aren't mentioned anywhere on the website - .avif will always be checked but it's only rarely used on websites.