Yes or no. Your assumption is correct and that more resources can be loaded in parallel by the browser if they are served from multiple domains. There are two cases to consider though:
Your server is not presently serving at maximum capacity: In this case more requests being served simultaneously is better. Visitors who's download bandwidth is not already maxed out will see reduced page-load time.
Your server is serving at maximum capacity: There will be little change, it may even slow down due to the overhead of getting and handling more requests. The way to speed up in this case is either to upgrade the server or use an actual CDN network.
You should look at your server performance logs to see which is the case. CPU usage is a good indicator when images are expected to be cached for the general case (you serve mostly the same images over-and-over). Disk usage is a better indicator when your image set is so large that it wont be cached so often (you serve different images to different users as for most personal gallery sites).