Hi Abel,
Your button can display any image stored in Pro's ArcGIS.Desktop.Resources.dll. These images are listed in the wiki page on your post.
You can display a Pro button image on your custom button by setting the largeImage and smallImage attribute in the config.daml for the button element like this code snippet below. The snippet below will display Pro's FolderConnection image on a custom button on the ribbon.
<button id="my_button" caption="My Button" className="myButtonClass" loadOnClick="true"
smallImage="pack://application:,,,/ArcGIS.Desktop.Resources;component/Images/FolderConnectionAdd16.png"
largeImage="pack://application:,,,/ArcGIS.Desktop.Resources;component/Images/FolderConnectionAdd32.png">
</button>
Another good resource to read for this is the Referencing images from ArcGIS Pro section in the ProGuide: Content and Image resources wiki.
The Pro UI is written using WPF (Windows Presentation Foundation) which is a graphical User interface designed by Microsoft. In Pro, your Add-in can create buttons, dockpanes, panes, ProWindow (dialog) etc using the SDK Item templates. Here are some resources that will help you get started with developing UI with Pro SDK:
1. We have various samples in the ArcGIS-Pro-SDK-Community-Samples repo that can help you.
2. The Pro SDK team has recorded many sessions on YouTube (from previous conferences) that will also help you.
3. On the Pro SDK website, there is a collection of tutorials that can also help: ArcGIS Pro SDK Tutorials
Thanks
Uma