How do I make those type of URL work again:

http://www.frenchguys.com/gallery/zen/i.php?a=Underwater/NewEngland2007&i=Lobster_FC.jpg&w=392&h=392

Change zen to zp-core.

I can't change every single old link on my web site to zp-core

Why are you linking directly to the images in the first place? If you want, you can change the places where ZENFOLDER is defined. However you will be on your own with that and will have to make the change for each time you upgrade zenphoto.

because Zenpress always linked directly to the images. Is there a way to configure apache maybe to redirect all /zen to /zp-core ?

Erik - sorry for the confusion with the directory name change. You can consider it the last real "beta" incompatibility.

I had the same problem with old URLs, and came up with a one-time fix to change them all. Run this SQL:

UPDATE wp_posts SET post_content=replace(post_content,'/zen/i.php','/zp-core/i.php');

Or if you prefer the rewrite URL route...
RewriteRule ^zen/(.*)?$ zp-core/$1 [R,L]

Note: the rewrite rule will not work with any strange filenames, nor with subalbums. Subalbums would require something recursive... I just did the SQL after I realized that.

That seems to have worked really well (SQL way). Thanks much !

Ouf ! thanks a lot for that command, I was feeling very anxious when I saw all my zenpress links broken...

Yes, the command works fine for me too. Very quick.

Feel much better now.