Can additional MyBlock icons be added to the EV3 palette of icons available in the MyBlock builder?
Go to the directory 


(for home edition) c:\Program Files (x86)\LEGO Software\LEGO MINDSTORMS EV3 Home Edition\Resources\MyBlocks\images\ 
(for edu version) c:\Program Files (x86)\LEGO Software\LEGO MINDSTORMS Edu EV3\Resources\MyBlocks\images\


And create a group of 3 icons for each new icon:


PolyGroup_NNN_NAME_Diagram.png [size 34x34] - visible in the diagram
PolyGroup_NNN_NAME_Palette.png [size 25x25] - visible in the command palette
PolyGroup_NNN_NAME_Palette_MouseOver.png [size 25x25] - visible in the command palette while moving the mouse over the icon


Where 


NNN is a next available number (98 icons already installed), so continue with 098
NAME - use some name


Restart the SW and open MyBlock builder, you will find the added icon.

It will not work, if you open your code on another EV3 installation without your modifications.