#1. First, install Squarespace ID finder and find the ID of Text Block. In my example, we will have:

#block-yui_3_17_2_1_1717078403666_23167

#2. Use this code to Custom CSS box

Replace Pixabay with your icon URL.

#block-yui_3_17_2_1_1717078403666_23167 h3:after {
    content: "";
    display: inline-block;
    background-image: url(https://cdn.pixabay.com/photo/2023/09/20/15/47/fish-8265114_1280.jpg);
    background-size: contain;
    width: 60px;
    height: 60px;
    background-repeat: no-repeat;
    background-position: center;
}
#block-yui_3_17_2_1_1717078403666_23167 h3 {
    display: flex;
    align-items: center;
    justify-content: space-between;
}

#3. Result