TinyPNG uses smart lossy compression techniques to reduce the file size of your PNG files. By selectively decreasing the number of colors in the image, fewer bytes are required to store the data. The effect is nearly invisible but it makes a very large difference in file size!
TinyPNG offer a cloud based service for shrinking PNG and JPG images. With an API key you can process up to 500 images per month for free (more if you pay for the service).
This would be a great plugin development project for anyone that’s keen 
Ideally the plugin would:
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have a config for nominating a developer api key
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bubble up an event whenever a source image is uploaded
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asynchronously process the uploaded source via tinyPNG
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overwrite the original source
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bubble up an event to re-cut any related images as needed