Hello and welcome to the community!
In your iconset.png file (inside the project's \img\system\ folder) the icons are numbered as follows:
The first row is made up of icons 0-15, the second row is 16-31, the third row is 32-47, etc...
Row 2 (icons 16-31) are the buff icons, in order: single buff mhp (16), mmp (17), atk (18), def (19), mat (20), mdf (21), agi (22), luk (23), then the same for double buff (24-31).
Row 3 (icons 32-47) are the debuff icons, same as before, single debuff then double debuff.
Whatever those icons are in your iconset.png is what the game will use. You can use any imaging program to edit the file, but it must be saved as a png file at \img\system\iconset.png. Each icon is 32 x 32 pixels, and there's no empty space between them.