Never used any of these services. I probably would not recommend using an external service on a live site. Zenphoto is file system based so you would have to transfer any image and any size needed by a theme to that service and then back to your Zenphoto site. I would guess that the benefit of smaller images kills the performance on that so you don't have any benefit. If it probably would have to be done while precaching.
I would recommend to do that before uploading since the most these tools do is strip meta data to achive "lossless" compression. There are open source tools for desktop like https://imageoptim.com/ (Mac only, has also an API/web service).
Also you can change the quality Zenphoto renders. If you have bigger images than needed in display and shrink them visually you can do higher compresssion. And then there are responsive images which is something different (we probably talk about that on other channels).
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I would recommend to do that before uploading since the most these tools do is strip meta data to achive "lossless" compression. There are open source tools for desktop like https://imageoptim.com/ (Mac only, has also an API/web service).
Also you can change the quality Zenphoto renders. If you have bigger images than needed in display and shrink them visually you can do higher compresssion. And then there are responsive images which is something different (we probably talk about that on other channels).