Thanks for the continuing updates.
The publication issue may be a complication but ultimately if the parent company commissioned the publication then they are vicariously liable for any infringement either of copyright or your personality rights. I doubt if the legal situation in Germany is substantially different on this.
As for the photo competition excuse, this is nothing but a red herring. Under UK law it doesn't matter how they obtained the image, if its use was not authorised by the rights holder, then that is infringement. This is entirely different to the defence (in UK law) of not knowing or reasonably believing that copyright existed in the image. Given the obvious recency of the image, it cannot be reasonably claimed that the company thought no copyright existed - ie that the image was out of copyright.
Their claim rests on their belief that copyright had been transferred to them, via the rules of the competition, which is a separate issue and not one which is recognised as a defence. No doubt your German lawyer can explain whether the same holds under German law.
Clearly the fact that the image doesn't appear amongst those submitted to the flickr group strengthens your case as it may indicate an attempt to mislead, unless of course they were referring to some other competition. In the UK courts this might be evidence of flagrancy which might lead to an award of increased damages. However, where and how they obtained the image is largely irrelevant for the reasons given above, as far as determining whether infringement has occurred.
If you are worried about infringement or your work has been copied and you want to take action.