update image usage guidance (#9198)

CONTRIBUTING.md

@@ -56,6 +56,13 @@ For more information, see the [DocFX Cross Reference](https://dotnet.github.io/d
 
 ## Images and screenshots
 
+Don't include images with articles, except:
+
+* In basic onboarding (beginner) tutorials.
+* When an image is needed for clarity.
+
+These restrictions reduce the size of the repository.
+
 As an optional step, ensure that any images and screenshots used in the documentation are compressed, which helps with file size and page load performance. A few popular tools include TinyPNG (using the [TinyPNG website](https://tinypng.com/) or the [TinyPNG API](https://tinypng.com/developers)) or the [Image Optimizer](https://marketplace.visualstudio.com/items?itemName=MadsKristensen.ImageOptimizer) Visual Studio extension. 
 
 ## Code snippets