I LOVE Articulate Storyline, I LOVE The Noun Project, and I LOVE creating easy value-add components for my client projects. Custom icons are something you can easily do to create value add for clients (everyone loves customization!), and The Noun Project Makes it very easy!
The way The Noun Project works is that artists submit their work/iconography, and if you have a free account, you can use the icons for free with attribution or pay a licensing fee. However, the paid account is only 39.99/year and WELL. WORTH. EVERY. PENNY! The paid account also provides some colour customization for the icons, and you get to support artists and a site that lets them share their work to a very large population. Win. Win. Win!
Check out the screencast below to see how easily you can use icons from The Noun Project to quickly create custom icons/slide objects in Articulate Storyline!