Microsoft CRM Custom Entity Logos 2013/2015

Hello, 

I have a custom solution which I am upgrading from 2011 to 2013 and 2015. I would like to swap the logos for my custom entities from the coloured logos in 2011 to the grey/white ones used for later versions. I have tried using some logos from the 'Microsoft Dynamics CRM\CRMWeb\_imgs' folder but they still don't look quite right. Has any had experience with creating their own or found a resource where I can download some?

Thank you for any help in advance.