Should I resize photos before or after image optimization?

Question

I am a software dev who appreciates stunning imagry in sites I build, but am also very conscious of page size. My workflow has me adjusting image sizes to the maximum resolution they will be displayed to the user. I also use ImageOptim a tool that applies a few compression methods to my images. I am wondering what is better though, to compress the images before I resize them, or after?

Answer 1

The "ImageOptim" tool pulls together a bunch of other things, and in the case of JPEG files, the relevant thing is the MozJPEG optimizing encoder. If you use this encoder and then resize and save with a different encoder, you will lose the benefit. Saving with the optimizer needs to be the last step.

Also worth noting: if you're starting with a JPEG and then saving as JPEG using a non-optimizing encoder and then saving again with MozJPEG, you are probably introducing artifacts. Those won't just reduce image quality — they may actually make the final image size worse. So, use a lossless format like TIFF or PNG as your intermediate. (PNG will lose metadata, but since you're throwing that away to get the smallest possible images anyway, that's fine.)