Image Optimisation

Not sure if this is considered SEO exactly, but I'm curious to know what everyone else does to optimise images?

1. How much do you drop quality? How much is too much?
2. What res do you use?
3. How do you deal with images for retina displays?
4. Do you size to exactly what your site requires or leave it larger?
5. Any other tips or tricks?
6. Whats a good optimisation level to aim for?

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I'm basically unsure if I've gone to optimise? I'm currently exporting my images from Lightroom with the following settings:

• JPEG
• Quality 25
• 800x600 pixels
• Resolution 80ppi

Images are roughly 60KB on export.

Glossy compression with ShortPixel drops finished images to roughly 40KB.