How to put background picture to phoenix html page?

You might use CSS for this…

If You look at the source code of https://elixirconf.com/ You will see

.main-hero {
  background-image: url(/2018/images/resources/jose-2016-keynote-256cbc4….jpg?vsn=d);
  background-position: 10%;
  background-position-y: center;
}
.fc-fullpage {
    background-size: cover;
    background-repeat: no-repeat;
    width: 100vw;
    display: flex;
    -ms-flex-align: center;
    align-items: center;
    -ms-flex-line-pack: stretch;
    align-content: stretch;
}

should i add this to my app.css?

Yes, and tag html accordingly.

i just want to center an image and have it on the bottom of the page, should i put it on the body tag the css tags?

bottom of page? You should use a wrapper instead of body… any div with corresponding classes will do the trick.

By the way it is also possible to use img tag with src pointing to assets, but using a background image is different because You still can put text on.

when i put stuff into app.css it saves first and changes the contenct in it form a lot of comments to a lot of tags, and then it chnages back, and my classes are gone

Are You editing the app.css in priv/? Because You should not, this one is autogenerated.

But without more info, (Phoenix version, umbrella?, bundler in use) it’s hard to tell.

yeah that was the issue, found the normal css file, but it still doesnt show up:
added this to css:

.background {
  background-image: url("<%= static_path(@conn, "/images/backgroundsuperadmin.png") %>");
  background-position: 10%;
  background-position-y: center;
}

and a div wrapper inside the body with this class name

That is css, not eex. AFAIK You cannot use <%= %>

Instead, use the real path… You should check also that going to the url returns the image correctly.