Add a line to the application manifest: fap_icon_assets="folder", where folder points to the folder where your icons are located
Add #include "application_id_icons.h" to the application code, where application_id is the appid from the manifest
Every icon in the folder will be available as a I_icon_name variable, where icon_name is the name of the icon file without the extension

Example

We have an application with the following manifest:

App(
    appid="example_images",
    ...
    fap_icon_assets="images",
)

So the icons are in the images folder and will be available in the generated example_images_icons.h file.

The example code is located in example_images_main.c and contains the following line:

#include "example_images_icons.h"

Image dolphin_71x25.png is available as I_dolphin_71x25.