With version 9 of Dynamics 365 Customer Engagement, Microsoft added a new type of image web resource : SVG files. They are vectorial images that can be used in any size. These images are used for Unified Client Interface, the new web interface also introduced in this version.
So, when customizing custom entities, there is now three possibilities to define icons: Small icon, Medium icon and Vectorial icon
Small icon: Used in the Solution Explorer and in the Lookup controls
This icon must have a size of 16 pixels by 16 pixels, transparent background and forecolor #5D5D5D
Medium icon : Used in the standard web interface Site map
This icon must have a size of 32 pixels by 32 pixels, transparent background and white forecolor
Vectorial icon : Used in the Unified Client Interface Site map
This icon must be a vectorial image with transparent background and black forecolor. D365 will adapt the color to render the icon properly.
You can find icon for your entities on www.flaticon.com, some icons are free, even in SVG format.