You have to make the Custom Resources folder.
Name your skin something different.
Creating a new skin with its own SmallIcons.png file in it does not work. Like it's been said, you have to create 2 new folders; Custom Resources and Custom Art.
Here's the procedure in this topic called "How to change the tree icons"
https://yabb.jriver.com/interact/index.php/topic,56830.msg925151.html#msg925151I implemented this in MC29 and it works. Not sure if it survives an MC update until we get another.
Here's the tip
For the stack icon list column
* Create a folder called "Custom Resources" in the "Data" folder you find in the MC program files directory.
* Create a new text file inside your "Custom Resources" folder and name it "Images.xml"
* Paste the following text into the Images.xml file and save the changes.
<Images>
<SmallThumbStackTopCollapsed>0</SmallThumbStackTopCollapsed>
<SmallThumbStackTopExpanded>1</SmallThumbStackTopExpanded>
<SmallThumbStackMember>2</SmallThumbStackMember>
</Images>
* Create a new image file 48 pixels wide and 16 pixels high.
* In the first 16x16 area of this file, create your "collapsed stack" icon.
* In the second 16x16 area of this file, create your "stack top, expanded" icon.
* In the third and final 16x16 area of this file, create your "stack member, expanded" icon.
* Save this file as "SmallIcons.png" in your "Custom Art" folder.
* Restart MC and it will use your custom icons in the list column.