How many concurrent AJAX (XmlHttpRequest) requests are allowed in popular browsers?
One trick you can use to increase the number of concurrent connections is to host your images from a different sub domain. These will be treated as separate requests, each domain is what will be limited to the concurrent maximum.
IE6, IE7 - have a limit of two. IE8 is 6 if you have a broadband - 2 (if it's a dial up).
The network results at Browserscope will give you both Connections per Hostname and Max Connections for popular browsers. The data is gathered by running tests on users "in the wild," so it will stay up to date.
With IE6 / IE7 one can tweak the number of concurrent requests in the registry. Here's how to set it to four each.
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings]"MaxConnectionsPerServer"=dword:00000004"MaxConnectionsPer1_0Server"=dword:00000004