Hello Lasse,
nice to meet you here and thank you very much for your detailed answer!
As luck would have it I'm currently reading a book about test driven development written by someone called Lasse Koskela

I'm just at the beginning but by now I have to say that this is definitely one of the very best books about this topic. In particular all the concrete practical examples are very helpful to get a feeling how to do test driven development in practice. I'm really looking forward to read the rest of this great book. Congratulations!
Your answer was very helpful, too. At least now i know, that it's not always possible to use only simple unit tests to test some functionalities. I guess the border between "real" unit tests and your so called integration tests is often somewhat blurry, isn't it? But I suppose it's inevitable to get some experience on my own to fully understand the concepts of test driven development.
Anyway your examples seem to be very similar to the example I've described and I'm glad that you solved the problem in a similar way than I intended to do. The idea to use real but small test images for the proof of concept sounds very reasonable to me.
Do you have a rule of thumb when you call some tests unit tests and when they are more likely integration tests? And how do you treat these two kind of tests differently? Just by running the integration tests less often? Are you using the
junit framework for both sorts of tests even so some are not real unit tests?
Of course I'd like to keep you up-to-date about this software project. The image recognition algorithm is just a part of my diploma thesis. There will be other interesting parts like creating 2D and 3D presentations of some data structures or controlling of a Lego Mindstorms robot which will definitely lead to interesting and difficult tests, I think. I would be very glad if I could get back to your help again then
Greetings
Marco