I am testing the new Image Serving upgrade. I modified my hosts file to point to your staging environment but now it shows missing images all over my site.
This outcome is expected. Our staging environment is not a mirror image for the production environment, so you may not see the recently added content. Normally, your production account is configured to also publish to staging. However, it could be inactive. And, it is also possible someone in your company is testing some functionality on SPS staging and publishing, which is overwriting the staging manifest. The staging SPS database is out of sync with production, and is only updated twice a year, so discrepancies can slip into tetsing.
However, testing an Image Serving upgrade concerns the actual functionality of the Image Serving / Image Rendering commands rather than images not displaying. So, for a certain set of commands applied, such as wid=300&op_sharpen=1, you want to double check are these commands actually working as intended.
Our QA team has already extensively tested the new release in our test environment. So, essentially, it is a way to double-check with live customer content.