Use ImageMagick to healthily compress JPEG images.

Compressing JPEG

• quality in 85
• progressive (comprobed compression)
• a very tiny gausssian blur to optimize the size (0.05 or 0.5 of radius) depends on the quality and size of the picture, this notably optimizes the size of the jpeg.
• Strip any comment or exif tag

convert -strip -interlace Plane -gaussian-blur 0.05 -quality 85% source.jpg result.jpg

Reference

• Stack Overflow post