fixed summary image filetype to jpg

[MagnusSmith]

Please add a direct link to your post here:

https://.github.io/blog/

Have you (please tick each box to show completion):

• Added your blog post to a single category?
• Added a brief summary for your post? Summaries should be roughly two sentences in length and give potential readers a good idea of the contents of your post.
• Checked that the build passes?
• Checked your spelling (you can use npm install followed by npx mdspell "**/{FILE_NAME}.md" --en-gb -a -n -x -t if that's your thing)
• Ensured that your author profile contains a profile image, and a brief description of yourself? (make it more interesting than just your job title!)
• Optimised any images in your post? They should be less than 100KBytes as a general guide.

Posts are reviewed / approved by your Regional Tech Lead.

[MagnusSmith]

https://magnussmith.github.io/blog/2024/12/18/taming-nullness-in-java-with-jspecify.html