Wrong path to sprites when using "domain.com/subfolder" as root

Dear people,

i got a local site im working on using "MAMP" on Mac. All Sites are organized in folders under http://localhost/[foldername]
Generating sprites is working fine, but the path always points to

http://localhost/sites (...)

instead of

http://localhost/[my-site-name]/sites (...)

I tried setting $base_url='http://localhost/[my-site-name]' in settings.php, but no luck. Any ideas what else i can do?

Thanks in advance,
F.