Now I have made a new application with the same pictures. The new complied Linux 64 Bit application looks good. The picture has the same size. I have checked this with a compiled version one made on a Mac and the other one made on Windows. I got the same result. All pictures a drawn correct.
I have deleted the pictures in my old project and made a new picture with the same graphics files. After I have compiled my app for Linux, I got the same wrong picture size.
After I have cleared the application cache, removed the complete build folder and compiled the app, I got the same image problem. When I check the folder "Resources", the image with the double size was renamed to "RedHat128@2x.png" and the normal size has the same name "RedHat128.png. But when I check the resolution for both pictures, I got 96 x 96 DPI.
Also I have checked my new Linux Test App. On mac the images in the resources folder has the same resolution of 72 x72 DPI. On Windows they have 96 x 96 DPI.