[CentOS] How to speed up Rsync transfers

Is there a way to speed up rsync transfers? I tested the bandwidth with iperf (recommended to me in an earlier post & worked well)its as advertised by my ISP's around 740KB/sec. When I manually run my rsync script with the --progress switch the transfers are around 100KB/sec. I googled this and the only thing I found had to do with the TCP window which I understand to be the limiting factor. But if this is true how can I ftp stuff at 300KB/sec? (someone please enlighten me)

I'm backing up jpg files and some days they add 5+GB of images. My goal was to backup the images nightly, but at the 100KB/sec rate that's not possible. Dan