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Thanks for your answers, I think the first has more importance for Google, as it is for the user. Too bad the study can't be found anymore !

I agree that you should be able to upload a big image, but unfortunately most of the libraries I've seen for doing backend image manipulation don't do a great job scaling down large images. They often end up blurry. Whereas a good graphics program will sharpen properly and you'll end up with a much better looking image. Would be interesting to see if yours looks better if you upload a pre-scaled one.