You can set it in your scss but you have to either have the file hosted somewhere or added in your mendix project images directory. C:\Users\xxxx\Documents\Mendix\xxxxx\theme\styles\css\images
If you do this then you need to create a class in your scss. This is mine for example:
.photo-bg{
background-repeat: no-repeat;
background-attachment: scroll;
background-position: center right;
background-size: cover;
align-content: center;
background-image: url("../images/blur-bg@3x.jpg");
background-image:
-webkit-image-set (url("../images/blur-bg.jpg") 1x,
url("../images/blur-bg@2x.jpg") 2x ,
url("../images/blur-bg@3x.jpg") 3x );
background-color: #064468;
.shs, h1{
color: #fff;
}
}
If it's from a hosted service you could add it into your scss as such:
.yourImage {
background: url("https://insertimagelocation.png") no-repeat;
background-size: contain;
height: 20px;
width: 20px;
}