Particularly, please make sure your image is no larger than 150KB. Using an image with a transparent background is encouraged. Please save your custom image in JPG, .BMP, .PNG or .GIF format.
Also, the dimensions of the image do not matter, though square images, or images that are wider than their length, tend to look best. Importantly, all phones have different screen dimensions, thus images will look different on different screens. Experiment and see what you like best.
In addition, for helpful tips and an in-depth look on branding, check out this article on our blog.