It is highly recommended to view the images full size at 100% zoom. Some images especially on mobile are scaled down by the browser, making it harder to spot the differences.
Visually lossless optimization
*Score shows how many images are visually indistinguishable from originals under normal viewing conditions. Hover over results to see the actual number of images. Additional tests revealed other issues.
ImageOptim scored 24/24 in lossy PNG compression but did not perform well on gradients and flat graphics in extended testing.
Tools like TinyPNG and Crunch use the original PNGQuant for color quantization, and generally perform worse than ImageOptim in terms of quality and lossless compression.
TinyPNG images could be losslessly compressed by 6.3% more using Optimage.
Guetzli achieved best quality/size ratio in some cases but failed to provide consistent quality especially at lower quality settings.
Guetzli is extremely slow requiring approximately 1 minute of CPU and 100 MB of memory per 1 MPix.