Yes I can reproduce here http://cutesoft.net/example/general.aspx.  It is not when you submit, it is when you switch from Normal to HTML view.

 

Steps to reproduce

1) In HTML mode, type the following - <img style="width: 374px; height: 348px" alt="" src=http://hp.msn.com/c/hotmail/en-US.gif />

2) Switch to Normal mode and notice image displays.

3) Switch to HTML mode, note the US in the URL is now lowercase us

 

Jason