The official, safe is hosted on the Paint.NET Forum Community Plugins section. As of the latest update, here is the direct path:
Paint.NET, a robust raster graphics editor, natively handles transparency via layer properties. However, it lacks a native, one-click "Normalize to Opaque" function for rasterized pixel data. This void is filled by the third-party plugin ecosystem, specifically the "Alpha to 255" or "Set Alpha" plugins. paint net alpha to 255 plugin download
Once you have the correct .dll file, follow these exact steps: The official, safe is hosted on the Paint
that specifically handle setting alpha values to 255 (full opacity). Recommended Plugins for Alpha-to-255 Tasks here is the direct path: Paint.NET