For a fan project, I would like to use the images of the investigators. As their use is (literally) quite big, I would like to know where highest resolution pictures of these silly ducklings can be found. Arkham Wiki? Fantasy Flight? Some other site?

Thanks!

Try Arkham Horror Wiki. They're not extremely high res, but they could work for you. Personally whenever I need anything I just scan it at 600dpi and use a noise filter, then bring it down to 300dpi. Works really well.