Images for iconbox and maybe even an icongrid

Yes – best would be to see the site you are talking about.

then – i would not set the background on the .iconbox_icon but substitute the before content.
where does the number come from? .iconbox_icon1 ???

you can replace before content by declaring it to “” empty:
and place then the background-image. You can play with height and width of that:

.custom-iconbox .iconbox_icon:before {
  content: "";
  background-image: url(https://path-to-your-image.jpg);
  background-repeat: no-repeat;
  background-size: contain;     /*** or cover is better if your image has 1:1 aspect ratio ***/
  background-position: center;
  width: 50px;
  height: 50px;
  display: inline-block;
  position: relative;
  transform: translate(-50% , -50%);
  left: 50%;
  top: 50%;
  border-radius: 50%;
}

Advantage is in additon that you can have with png files the background-color of your theme.

Sorry my mistake that the custom class goes to the container I missed:

 .avia-icon-grid-container.custom-icongrid li:nth-child(1) .icongrid-char:before {
  content: "";
  background-image: url(https://bp.domain.com/wp-content/uploads/2019/08/1-Start.png);
  background-repeat: no-repeat;
  background-size: contain;
  background-position: center;
  width: 50px;
  height: 50px;
  display: inline-block;
  position: relative;
  border-radius: 50%;
}