Replace blog images with optimized JPG versions: 90% size reduction

- Converted all blog post thumbnails from PNG to JPG format
- Resized from ~1500px to 600x400px optimal dimensions
- Total blog images: 5.73MB  0.6MB (90% reduction!)
- Individual improvements:
  * responsive-school-website-design: 736KB  59KB (92% reduction)
  * Futurelab Solutions Blog Post: 677KB  65KB (90% reduction)
  * landing-page-vs-website: 658KB  74KB (89% reduction)
  * truth-about-website-builders: 655KB  89KB (86% reduction)
- Updated all file paths in blog.html to use new JPG files
- Removed old PNG versions
- Blog page should see massive performance improvement

Author: Sam-May-Futurelab