Did you do any modifications in the code?
For some reason your site returns two sets of cookies when you try to login:
Set-Cookie: 4images_lastvisit=1282185938; expires=Fri, 19-Aug-2011 02:49:28 GMT
Set-Cookie: 4images_userid=-1; expires=Fri, 19-Aug-2011 02:49:28 GMT
Set-Cookie: 4images_userpass=deleted; expires=Wed, 19-Aug-2009 02:49:27 GMT
Set-Cookie: 4images_lastvisit=1282186157; expires=Fri, 19-Aug-2011 02:49:28 GMT
Set-Cookie: 4images_userid=1; expires=Fri, 19-Aug-2011 02:49:28 GMT
Note the line with 4images_userpass that is exactly what your server returns, I didn't alter that. That is not something 4images does, it look like you've changed something in the code.
When you login at normal login page (not ACP) and selected "Log me on automatically next visit?", then you stay logged in fine, and that is the cookies server returns:
Set-Cookie: 4images_lastvisit=1282184897; expires=Fri, 19-Aug-2011 02:45:38 GMT
Set-Cookie: 4images_userid=-1; expires=Fri, 19-Aug-2011 02:45:38 GMT
Set-Cookie: 4images_userpass=21232f**************801fc3; expires=Fri, 19-Aug-2011 02:45:38 GMT
Set-Cookie: 4images_lastvisit=1282185604; expires=Fri, 19-Aug-2011 02:45:38 GMT
Set-Cookie: 4images_userid=1; expires=Fri, 19-Aug-2011 02:45:38 GMT
Again, it returns two sets...