How to change safari Favicon on ios 7 (Safari speed dial images )?

You cannot change these images. If there is an iMage Safari can pull from the website, like when adding Apple.com you get the grey Apple, Safari will pull it. Otherwise, Safari will set the default image and randomly choose a color, no way to change the image nor color.

Give this app a look.

https://itunes.apple.com/us/app/app-icons-free-change-home/id749073889?mt=8

I used it once a while ago. If I remember correctly the Safari icons you change end up on your home page. Another thing you could try when you bookmark a site as a favorite choose "add to home screen" when available. Usually you'll get a much better looking icon.