This solved the problem!
Thanks a lot!!!!!
I switched to method “Retina-Images” now.
Thanks. I will try that.
I tested the website on different retina devices.
The problem didn’t appear on an iPad 4 (,Google Nexus tablet & Samsung Galaxy S3).
I only had those problems with an iPad Air and an iPhone 5s.
I will let you know if I have news about my problem….
Thanks for your help!
Thanks for your fast answer!
I chose the method “Picturefill” as recommended.