Use png format instead of jpeg

Changing the format from jpeg to png allows transparent backgrounds, which eliminates the issue of black backgrounds when using the jpeg format if the PDFs are transparent.

Another patch provides a solution to this problem, but it bypasses the Spatie PDF to Image library which has a number of advantages with the options it provides. As such, it would be ideal to keep using that, otherwise this module should probably provide a more comprehensive implementation of it's own imagick thumbnail generation function that can take care of the various options that may be useful.