For my aricles (wp7app.de) I'd like to use two different thumbnails, one which is displayed only on the index page (150x150) and one for the article page (600x400). However, I don't want to use a cropped version of the big image for the index. Is this something that can be done easily without an extra plugin or can you recommend a good / simple one? Cheers
You could create a template for the pages that you want a bigger thumbnail for and call a bigger thumbnail in the code of that template.
I ended up using the "Multiple Post Thumbnails" plugin which I'm quite happy with: https://wordpress.org/plugins/multiple-post-thumbnails/