I know its a bit late for an answer, but maybe someone find this useful. I use this on my CSS styles to change the button background. Just change the url to your image actual path.
[code:24jjvm2k].sf-with-ul {
background: url("your_images_folder/some_background_image.png");
}[/code:24jjvm2k]
To change the character itself its a bit more tricky, in the pixopoint plugin folder, inside scripts look for a file called superfish.js, then look for the number 187 (wich is the ISO character entity reference number for the right double arrow), if you want another character instead the double arrow try another number*. But if you need to wipe it and use an image instead, you can use the blank-space reference number (167) type it, save the file and add this code to your CSS.
[code:24jjvm2k].sf-sub-indicator {
display: inline-block;
width: 10px;
height: 10px;
margin-left: 3px;
line-height: 15px;
background: url("your_images_folder/some_bullet_img.png") no-repeat;
}[/code:24jjvm2k]
*FYI this is the list for ISO entities numbers.
http://www.w3.org/TR/REC-html40/sgml/entities.html
Cheers.