I found the problem! Thanks for your hints!
My camera, although in many respects very good and equal to a beginnr’s DSLR, under certain circumstances has a problem with quality. It doesn’t occure at full aperture. But the smaller the aperture, the lower the resolution 😦 So, until I can afford a DSLR I’ll have to try to circumvent this. I won’t do that with my cell, don’t worry, but it’s nice to know that I can use the pictures on the blog, too. Here is another one 😉