Since you have it set to skinid, it is going to use the skinid of the ability, not the URL.
"Extra pockets": {
"enabled": true,
"max_level": 5,
"tier": 2,
"value_per_buff": 2.0,
"buff_info": {
"Key": "ExtraPockets",
"Value": "Slots"
},
"icon_url": "https://www.dropbox.com/s/damaguptw7r54r5/Extra_pockets.png?dl=1",
"skin": 2873047443, <-----------
"permissions": null
},
You will need to make the image 512x512 and us a tool to upload the image to the steam workshop. I recommend custom icon creator by Billy.