Hi,
I don't understand what you are trying to do, but follow these;
1- Open the Dropdown Shopping Cart Module backend
2- Select the default icon (icon-1)
3- Then find this PNG file: mod_flexible_vm3_cart/assets/images/icon1.png
4- Open this PNG image with Photoshop
5- Insert your own icon into that image file (without changing the image resolution)
6- Save the file with the original name (icon1.png -- overwrite with the existing file)
7- Then everything should be good to go.
FYI: Icons should have PNG file format. Because JPEG doesn't provide "transparency" for the background
Regards.