Labels can't use toolbar IconSets. They need standalone icons (e.g. .ico files, or icons inside a .exe or .dll file).
(The Express Dual IconSet you found uses a single PNG image grid for each icon size. You can inspect it by renaming the .dis file to .zip and extracting it. To use an image from that on a label, you'd need to copy the icon bitmap out of the PNG grid and into a standalone .ico file using an icon editor. An easier solution is to find a standalone .ico file that has the image you want.)
A much easier solution: Use icon 15 inside /system/imageres.dll: