some kind of icon desappear when restart the machine

[Panda]

I wrote the file `~/Desktop/.LibreSprite.desktop` with the contents:
```
[Desktop Entry]
Version=1.0
Terminal=false
Type=Application
Name=LibreSprite
Exec=/home/panda/LibreSprite.AppImage
Path=/home/panda/Pictures
Icon=/home/panda/Pictures/icons/libresprite.ico
Categories=Utility;Development;
```

And then I grabbed the file and dropped in the Plank menu, alright, it worked fine.
But when I restarted my computer and logged in again, the icon desappeared!

My plank version: 0.11.89
My linux distro: Ubuntu 22.04.2 LTS (Linux 5.15.0-71-generic)
Window manager: XFCE