To replace mobile menu items with custom image, you can use this code to Custom CSS box
[data-folder="root"]>div>div>div a {
background-size: contain !important;
background-repeat: no-repeat !important;
background-position: center center !important;
color: transparent !important;
}
[data-folder="root"]>div>div>div:nth-child(1) a {
background-image: url(https://static1.squarespace.com/static/62ca90b4357da95817cf4a70/t/62d1253b9ad3ff22ceb82286/1657873723478/arbeiten.png);
}
[data-folder="root"]>div>div>div:nth-child(2) a {
background-image: url(https://static1.squarespace.com/static/62ca90b4357da95817cf4a70/t/62cfcc20b0042e2b435fb69f/1657785376772/CV.png);
}
[data-folder="root"]>div>div>div:nth-child(3) a {
background-image: url(https://static1.squarespace.com/static/62ca90b4357da95817cf4a70/t/630f00e4289a9e107070cf8f/1661927652894/Bildschirmfoto+2022-08-31+um+08.34.28.png);
}
result
