Thank you all a lot!
I wish I could figure out what is wrong with these images. They are part of a larger collcetion of handdrawn old west-images. I had the hope to use it for other purposes, because they are handdrawn vectors in eps-format. Maybe they are results of traced/vectorized handdrawn sketches and the reason is caused by that anyway. I had never such a problem before with vector-images. But making a bitmap copy is a way to work with the images.