What does this PR do?

Image's onLoad prop is not provided to an Img tag

Where should the reviewer start?

src/js/components/Image/Image.js

What testing has been done on this PR?

yarn test

How should this be manually tested?

1. You can change the Image storybook story in src/js/components/Image/stories/Simple.js to:

export const Simple = () => (
  <Image
    src="//v2.grommet.io/assets/IMG_4245.jpg"
    onLoad={() => console.log('onLoad')}
  />
);

2. Run the Storybook,
3. Go to the Image Simple story
4. Open the browser console to check if the onLoad message appears

Do Jest tests follow these best practices?

• screen is used for querying.
• The correct query is used. (Refer to this list of queries)
• userEvent is used in place of fireEvent.
• asFragment() is used for snapshot testing.

Any background context you want to provide?

What are the relevant issues?

#6202

Screenshots (if appropriate)

Do the grommet docs need to be updated?

No

Should this PR be mentioned in the release notes?

Is this change backwards compatible or is it a breaking change?

Backwards compatible