Here are some guidelines to follow for promoting apps on your Web Site or Facebook page for example, including app store badge generators and device art generators.
Apple guides:
https://developer.apple.com/app-store/marketing/guidelines/
https://developer.apple.com/app-store/marketing/guidelines/#downloadOnAppstore
https://developer.apple.com/app-store/marketing/guidelines/#images
Google guides:
http://developer.android.com/distribute/tools/promote/brand.html
http://developer.android.com/distribute/tools/promote/badges.html
http://developer.android.com/distribute/tools/promote/device-art.html