Busy crafting day! Made 20 (!!) shirts for the boys and the new school year. All these images are free open source from The Noun Project https://thenounproject.com/.
There are four ways to correctly use an icon from The Noun Project. Please pay particular attention to #2, below.
1. You can sign up for a free Member account. Then you can download each icon for free. If the icon is listed as “public domain” then you can use it royalty free, without the need to give credit to the icon creator. In other words, these types of icons can be used on t-shirts without any other requirements.
2. You can sign up for a free Member account. If the icon is listed as being licensed under “Creative Commons”, when you download the icon you will click the “Free and Give Credit” button. You can then use the icon royalty free. You must, however, give credit to the icon creator. The Noun Project’s web site says the proper way to give credit is to include a statement in the form of “Icon name” by Creator Name, from thenounproject.com. For example, if you want to use a Tree icon that was created by Edward Boatman, you should use this statement: “Tree” icon by Edward Boatman, from thenounproject.com.
This statement must be listed next to the icon, below the icon, or somewhere on the project you create to properly reference the icon.
Taking these rules in mind, if you use one of the icons as an image on a shirt, you should attach a tag to the shirt. Somewhere on the tag you should include the statement (for example): “Tree” icon by Edward Boatman, from thenounproject.com.
3. You can sign up for a free Member account and pay $1.99 for each icon. This gives you the right to use it royalty free, without the need to give credit to icon creators. When you are logged in as a Member and find an icon you like, you click the “Purchase Royalty Free” button.
4. You can sign up for a NounPro account. This costs $9.99 each month. A NounPro account allows you to download an unlimited number of icons and use them all royalty free, without the need to give credit to the icon creators.
The details for these rules are listed at: https://thenounproject.zendesk.com/hc/en-us/articles/200509928-How-do-I-give-creators-credit-in-my-work-