Right now there is no way to test it but we backup all images to Amazon S3 and you can restore them just in case something goes wrong.
For compression we are using stable and tested open source tools like Mozilla JPEG Encoder Project
Also, when you install, you'll have free tier - 20MB of compression, so it will stop compressing after 20MB is used.
In dashboard you can see which images are compressed and if you are happy with it I can switch you to 2GB plan that will continue compressing your product images and also compress your asset images.
Hope that helps.
Dom