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