I know that neither the os nor the browser should make a difference... but it does!
If you got time please check again the url, click on "marrons" section for exemple (this one only content the image supposed to be the "albumthumb"), you shoud see something in FF and nothing in safari...
(i'm using Safari 5.0 and FF 3.6.6 )
Hm, I am sorry, I don't see anything in Safari 4.1 (my Mac is a little older currently so I can't test 5) and Firefox 3.6.6 (and yes, at least Firefox seems to behave differently on Mac and Win at times sadly - damm browser wars). I even tried Opera 10.6 with the same result.
Anyway, since you use some JavaScript for that image swapping this is not really a Zenphoto issue but one of that script I suppose.
oups... The problem just came from the fact that images specified invisible via the checkbox are still shown when the admin is logged in... and I was logged in with FF.
I'm sorry to have wasted your time (and mine too) with an issue that was not...
Thank you for all.