The two things to watch for while adding a custom image to a button is:
There is a ProGuide Diagnosing ArcGIS Pro Add ins that discusses this. Check out the
ArcGIS Desktop SDK team
To see where the problem lies, you could try this:
1. In Visual Studio, create a simple add-in with just a button item using the SDK Templates.
2. In your file explorer, access the Images folder of this add-in on disk.
3. You will see the GenericButtonBlue32.png and GenericButtonBlue16.png.
4. Delete them.
5. Copy your custom png to this folder and rename your png to be GenericButtonBlue32.png and GenericButtonBlue16.png
6. In Visual Studio, compile your add-in project and test if the button loads in Pro with the custom png instead of the generic blue image.
If it does, then you could check your add-in where it is not working. If it does not work, it could be a png issue.
When you are typing up a message in GeoNet, click on the "Use Advanced Editor" on the top right corner above your message.
You will see the "Attach" option at the bottom. Like this screenshot below.
The "Use Advanced editor" button is highlighted in the screenshot below. When you click that, you will see an option to Attach files to your message.