Update responsive_images.md: Add to additional resources section

[jbb-codes]

Added additional resources that help explain responsive image concepts better.

Because

These resources helped me grasp using both img and picture elements with the srcset and sizes properties.

This PR

• Added resources to the 'Additional Resources' section.

Pull Request Requirements

• [ x] I have thoroughly read and understand The Odin Project curriculum contributing guide
• [ x] The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• [x ] The Because section summarizes the reason for this PR
• [x ] The This PR section has a bullet point list describing the changes in this PR
• [x ] If this PR addresses an open issue, it is linked in the Issue section
• [ x] If any lesson files are included in this PR, they have been previewed with the Markdown preview tool to ensure it is formatted correctly
• [x ] If any lesson files are included in this PR, they follow the Layout Style Guide