See the documentation about assets here: https://platform.uno/docs/articles/features/working-with-assets.html

Here is a cheat sheet:

1. Add the image file to the Assets directory of a shared project.
2. Set the build action to Content.
3. (Recommended) Provide an asset for various scales/dpi

Examples

\Assets\Images\logo.scale-100.png
\Assets\Images\logo.scale-200.png
\Assets\Images\logo.scale-400.png

\Assets\Images\scale-100\logo.png
\Assets\Images\scale-200\logo.png
\Assets\Images\scale-400\logo.png

Table of scales