Yes it is Publisher indeed, I'm so sorry for the mix-up (Designer... InDesign, you know). And thanks for your time!
You're absolutly right actually: I did another set of exports a few minutes ago using the default print or export settings, and it looks like what you attached above. Using the internet/screen presets however gave the kind of result I posted in my first message. Increasing the DPI changed the result to something pretty close to what's produced using the other default settings. What I don't understand is why the results were almost identical, whatever I did, last time I tried. Maybe I missed something but still, I'm intrigued.
Also (even if sligthly off the question): shouldn't the export using the internet/screen default presets provides a non-pixelated result at 100%? Because it seems to me a bit useless to limit the pictures downsizing to, say 150 DPI, if is supposed to meet screen requirements.
As for the last part of your answer ok, this is a fairly logical process. I stumbled across the issue while testing Publisher possibilities, so now the way this works is a lot more clearer to me.