Add information about sizes and formats

docs/src/main/asciidoc/extension-metadata.adoc

@@ -51,7 +51,7 @@ metadata:
 <5> Categories the extension should appear under on https://code.quarkus.io[code.quarkus.io]. This field can be omitted, the extension will still be listed on https://code.quarkus.io[code.quarkus.io] but won't be categorized
 <6> Maturity status that could be `stable`, `preview`, `experimental`. It is up to extension maintainers to evaluate the maturity status and communicate it to the users
 <7> Link to the extension guide or documentation page
-<8> Link to an externally hosted image. This is used in the Quarkus dev tools as the extension icon. Alternatively, if you https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/customizing-your-repositorys-social-media-preview[set the social media preview] on your extensions source code repository, the tools will pick up that image.
+<8> Link to an externally hosted image. This is used in the Quarkus dev tools as the extension icon. It should be square, and any resolution greater than 220 pixels. Supported formats are png, jpeg, tiff, webp, and svg. Alternatively, if you https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/customizing-your-repositorys-social-media-preview[set the social media preview] on the extension's source code repository, the tools will pick up that image.
 <9> https://quarkus.io/guides/extension-codestart[Codestart] information
 <10> Configuration prefix