I hope you are well today and thanks for asking!
Which file format image do you have? Rating Manager supports Scalable Vector Graphics (SVG). If your image is JPEG/PNG, then you’ll need to use some converter, for example: http://image.online-convert.com/convert-to-svg or PhotoShop.
When you have the right SVG file, then you need to upload SVG source to Rating Manager > Style > Custom SVG.
To change the size of the icon (stars), go to Rating Manager > Style > Icon size.
Do you have demo setup? If so, then we would be happy to take a closer look.