can't upload header logo

I have created the embryo of a site called snap-decision.eu using the zippy theme. I am trying to upload a header picture and it appears to upload correctly, but the next stage is select and crop. Everything I try brings up an error “there has been an error cropping your image”. I have resized the image and tried both jpg and png formats.
Meanwhile the site title says Zippy and no amount of editing the title will change this, although the tagline is showing correctly. Please could you advise what might be wrong for both graphic and text versions.