A silly question to set icon

[lVentus]

I am writing a launchpad. I use the code in example to get appItems.
I found some appItem's icon name is a path to a png file or svg file.

Unknown icon format for app: QQ, icon: /nix/store/shclzi1bx71166x6kjxiwacj52wa7jxr-qq-3.2.9/share/icons/hicolor/512x512/apps/qq.png

Then the icon will be a pixel_size = -1; Even I set the pixel size, it won't work. How should I properly make the icon?
Thanks!