How do I optimize my images to reduce their size?

I’m currently using WordPress Theme Eleven-Eleven version 3.6 and I haven’t uploaded an image optimizing plugin yet. I only have the “Regenerate Thumbnails” plug-in and tested it on an image needing to be compressed per Google Analytics. It still remained as an image needing to be compressed.

I’m trying to figure out what would best help me “losslessly compress” my images, according to Google Analytics, by reducing their size by 178.3KiB (11% reduction). My site is super slow.

I would also like to ask which plugin would you recommend that would best help me with this? And, another question is does it change the image on the post, when using a plugin to optimize and compress these images?

My URL is http://plaintalkandordinarywisdom.com