When we asked to mail postcards back to the US, we found a new stamp in 2012. It's a small purple stamp with a stylized "Marianne" bust on it, which bears the value "20g", which I take to mean 20 grams. When we bought more in the Azay-Le-Rideau La Poste, the clerk charged 0.20 Euros, which I think was too little. What is their current price, and are they really the right stamp for small photo postcards from France to the U.S.? None of our concierges could enlighten us.
I can't tell you how much it costs to mail a post card from France, but we mailed one from Germany this past May and it cost 1.20 Euro.
Yes, that 20 gram stamp with "Marianne" is for mailing a post card from France to the US and the world (the word Monde is on the stamp) is the correct one. Both in France and Germany the important initial weight is 20 g. The postage amount for a postcard from Germany to the US is 75 cents (0,75 Euro). Get those in groups of ten called a "Marken Set" for 7,50 Euro.
It should cost .89€ to mail a postcard from France to the US. If I'm picturing the stamps you're describing correctly, they are not international stamps, but are last year's stamps for mailing a letter within France. The newest "1st class" equivalent stamp for within France is green. But it's hard without a picture to know if we're talking about the same stamps or if you have one I've never seen before.
They're the ones you want for postcards; just used them this past month; I think they were about $0.95.