Short answer
JPG compression can reduce file size substantially for photographic images, but the right setting depends on the original quality and the displayed size.
When this applies
Use it for photos, screenshots with photographic detail, and article imagery. Inspect faces, gradients, fine text, and sharp edges after compression.
Practical review steps
- Start with the original file and a realistic display size.
- Compare the output visually before publishing it.
- Use a different format or keep the original if artifacts are visible.
Check the actual color pair
Use the free Image Squeeze with the foreground and background colors from the real component. It provides an immediate AA and AAA ratio check in the current browser tab.
Source and limit
For the formal requirement, review W3C's WCAG contrast minimum guidance. A ratio result checks a color pair; it is not a complete accessibility audit.