1. We've found CB 1.2 Stable does not play nice with PHP4.
That was it ! The server was running php 4.4.8 by default, but my host also offers php5. The support team said that I could either rename all .php files in .php.5 (which I will certainly not do), or add a [code:1]SetEnv DEFAULT_PHP_VERSION 5[/code:1] in my .htaccess file. I did the latter and it solved both problems : the button images reappeared and javascript is now functional again.
What is weird is that 1.2 RC4 worked like a charm, even on php4. Modifying the required php version on a RC sounds a bit like a bug to me... Should I report it ?
Also, do you think that I have to "tell" some other parts of Joomla that I switched to php5 ?
Thanks for the tip anyway. I was quite stuck !