By looking at html code behind your login page, look like the dnn:commandbutton did correctly generate the image. Check your signin.ascx to see other code that might generate the link "Login". Are there any asp:Linkbutton or asp:button that might have done this?
Below is the code behind:
<input type="image" name="dnn$ctr381$Login$Login_DNN$ctl04" title="Login" src="/Portals/_default/Skins/ClassCampers/button-login.gif" alt="Login" onclick=" WebForm_DoPostBackWithOptions(new WebForm_PostBackOptions("dnn$ctr381$Login$Login_DNN$ctl04", "", true, "", "", false, false))" style="border-width:0px;" /> ->>GENERATE BY DNN:COMMANDBUTTON
<a class="CommandButton" href=" WebForm_DoPostBackWithOptions(new WebForm_PostBackOptions("dnn$ctr381$Login$Login_DNN$ctl05", "", true, "", "", false, true))">Login</a> ->>GENERATE BY ASP:LINKBUTTON?