Hi, the icon designer is meant as a mini-editor, and it’s based on SVGs.
It’s not as full-featured as a dedicated icon editor, but there are lots of web-based editors that will work. One of the easiest is Google’s material icons: https://fonts.google.com/icons (unfortunately, they don’t have a ‘waterfall’ icon either). Maybe a site like this will work for you: https://icons8.com/icon/eX1rkIM7LjBM/waterfall.
Or, try chatGPT. It whipped up a very nice icon for me with this prompt: “create a 128×128 png icon of a waterfall in the style of Google material icons”.
Anyway, once you have created a custom PNG file it’s easy to use it in MapPress and display it on your map (see the documentation for details).