Actually I’ve made it works properly by using code in https://codelabs.developers.google.com/codelabs/your-first-pwapp . I hope this question and answer helps other people if they have the same issue.

I’m working on a test project to start learning about service workers and progressive web apps.

This project is hosted on github in https://github.com/Pablo-No/Privado1, however, when I download it as a progressive web app in a mobile phone some images aren’t loaded correctly and alt text is shown instead, I don’t know how to fix it, probably because I don’t know how service workers and progresive web apps work, and that’s why I’m working on this project.

In my computer it works perfectly, even when the app is downloaded without connection to internet (thanks to the service worker, I think). I’ve seen some similar issues, but I think they’re different, however I may not be correct and I’m sorry if this issue has been answered yet.

I’ve used Chrome and Chrome mobile with the desktop and the mobile phone (the mobile phone uses android).