Images not appearing in Builder or Proof
If they are, this issue is likely due to your browser settings not allowing Mixed Content/Unsecure Content (the HTTP images in this case). It's possible these rules were applied over time by your IT team, or just general browser updates causing changes to those rules. Litmus does not block mixed content on our end.
In order to view those images, you need to either ensure Mixed Content/Unsecure Content is allowed at the Site Settings level (in Chrome, click on the lock next to the URL in the browser bar > Site Settings > Insecure Content > Allow), admin policy level and if that does not resolve the issue or it's not possible, you will need to work with your IT team to host your images as HTTPS/ensure the SSL cert for those images are applied correctly.
Before making changes to your Site Settings, please consult your IT team as they may want to review any changes to the security settings of your browser instances.