In the DAML for a Add-In you can define the png image to use for a tool.
<tool id="BasicMapTool_BasicMapTool" caption="Show Coordinates" keytip="T1" className="BasicMapTool" loadOnClick="true" smallImage="Images\crosshair16.png" largeImage="Images\crosshair32.png" condition="esri_mapping_mapPane">
Generally if you stay with one of the ESRI default icons they will appear. However, based on the pathing any 16 and 32 pixel png image referenced at .\Images should be usable. Where is this location? I've tried deploying them to the assembly cache (e.g., C:\Users\<user id here>\AppData\Local\ESRI\ArcGISPro\AssemblyCache\{ff226987-8efd-4d34-b606-3e731c8cd2ad}\Images) but that does not appear to work.
In Visual Studio 2013 it the setup looks like this (see attached screen shot_.
Rich D.