This is one I've got running that seems to work - try that and see if it doesn't make a difference.
You'll have to decipher some of the elements though - eg. widths, heights.

<dnn:MENU runat="server" id="dnnMENU"
separatecss="true"
display="vertical"
rootmenuitemcssclass="MainMenu_Off"
rootmenuitemactivecssclass="MainMenu_Active"
rootmenuitemselectedcssclass="MainMenu_Selected"
rootmenuitembreadcrumbcssclass="MainMenu_BreadcrumbActive"
delaysubmenuload="true" />

/*===================================================*/
/* Begin  Main Menu    - Solpart Menu Functionality  */
/* Tested in FireFox/Mozilla - Netscape 6, IE7       */
/*===================================================*/

/* Container for main menu */
.MainMenu_MenuContainer {
background: transparent;
padding: 0px;
width: 174px;
}

/* Menu bar */
.MainMenu_MenuBar {
padding: 0px;
background: transparent;
width: 100%;}

/* Applies to non active state of menu  */
.MainMenu_MenuItem {
font-family: tahoma; 
color: #999999;
font-size: 11px;
background: #F4F3F3;
font-weight: bold;
font-style: normal;
padding: 0px 10px 0px 10px;
height: 25px;
border: 0;}

/* Hover state for submenu items */
.MainMenu_MenuItemSel {
font-family: tahoma;
font-size: 11px;
font-weight: bold;
cursor: pointer;
cursor: hand;
color: #ffffff;
background: #96B3BB;
border-left: #7D9D92 0px solid;
border-right: #7D9D92 0px solid;
 padding: 0px 10px 0px 10px;}

/* Container for submenu */
.MainMenu_SubMenu {
z-index: 1000; 
cursor: pointer; 
cursor: hand;
background: #FAFAFA;
padding: 0px;
border: 1px solid #dddddd;
font-weight: normal;
filter: none;
padding: 1px 1px 1px 1px;}

/* Hidden Elements - This stops the icon and arrows from being displayed in the menu */
.MainMenu_MenuArrow, .MainMenu_RootMenuArrow, .MainMenu_MenuIcon, .MainMenu_MenuBreak {display: none;}

/* Non Selected state for main menu items */
.MainMenu_Off {
color: #6F7984;
background: transparent url(BusinessPix/Button_Off.gif) no-repeat ;
padding: 0;
height: 25px;
font-weight: bold;}

/* main menu items Selected */
.MainMenu_Selected {
color: #7D9D92;
background:  url(BusinessPix/Button_Act.gif) no-repeat;
padding: 0;
height: 25px;
font-weight: bold;}

/*  main menu item which is active */
.MainMenu_BreadcrumbActive {
color: #7D9D92;
background: transparent url(BusinessPix/Button_Ovr.gif) no-repeat;
padding: 0; 
height: 25px;
font-weight: bold;}

/* Over state for the active main menu items  */
.MainMenu_Over, {
height: 25px;
padding: 2px 2px 2px 0;
background: transparent url(BusinessPix/Button_Ovr.gif) no-repeat;
color: #67848A;}

/* active state for the active main menu items  */
.MainMenu_Active, {
height: 25px;
padding: 2px 2px 2px 0;
background: transparent url(BusinessPix/Button_Ovr.gif) no-repeat;
color: #67848A;}

/*==================*/
/* End Main Menu    */
/*==================*/

Here is what I had to do, in addition to changing the order.  Not sure this is very intuitive, any explanation would certainly be helpful and could provide learning for others.  I noticed few others with same trouble in forums.

(1) I changed the order of CSS items, per suggestion
(2) I simplified the solpart menu coding, eliminating unnecessary effects.
(3) I moved all nav images into their own subfolder
(4) I added the word "transparent" before the background URL in the CSS.  Nina, not sure why you didn't put the word transparent before the MainMenu_Selected class but things work.
(5) I moved font coding after the background URL tag in all CSS blocks
(6) I added the .MainMenu_Over class, even though it is not referenced from solpart menu control in Nina's code example.

<dnn:SOLPARTMENU
            runat="server"
            id="dnnSOLPARTMENU"
            separatecss="true"
            display="Vertical"
            userootbreadcrumbarrow="false"
            usesubmenubreadcrumbarrow="false"
            rootmenuitemcssclass="MainMenu_Off"
            rootmenuitemactivecssclass="MainMenu_Active"
            rootmenuitemselectedcssclass="MainMenu_Selected"
            rootmenuitembreadcrumbcssclass="MainMenu_BreadcrumbActive"          />

/*-- menuitems --*/
.MainMenu_Off    { color: #666; background: transparent url(NAV/nav_idle.gif) no-repeat; padding: 0px; height: 20px; font: bold 11px Tahoma, Arial; }

.MainMenu_Selected          { color: #666; background: url(NAV/nav_hover.gif) no-repeat; padding: 0px; height: 20px; font: bold 11px Tahoma, Arial;  }

.MainMenu_BreadcrumbActive  { color: #666; background: transparent url(NAV/nav_active.gif) no-repeat; padding: 0px; height: 20px; font: bold 11px Tahoma, Arial; }

.MainMenu_Over     { color: #666; background: transparent url(NAV/nav_hover.gif) no-repeat; padding: 0px; height: 20px; font: bold 11px Tahoma, Arial;  }

.MainMenu_Active    { color: #666; background: transparent url(NAV/nav_active.gif) no-repeat; padding: 0px; height: 20px; font: bold 11px Tahoma, Arial; }