See the "For Artists" tab for more information.
When looking to incorporate others' work into your own creations, think about how you would like your artwork to be treated. At the very least, you probably want it attributed to you. Even though students have a lot of leeway in using copyrighted material for educational purposes, it is good practice to become accustomed to complying with copyright restrictions for your own professional practice.
There is no set amount that is "ok" to borrow from an original work, nor is there a set amount of changes that must be made to the original before it can be used without copyright infringement. If making a Fair Use argument about using another's work, it is important to keep the fourth factor in mind: how does your use affect the market for the original? If you are using the original artwork in a way that is similar to how it was designed to be used, you are likely damaging the profit the original artist could have made from the work.
If you're not sure if the work you want to use is still under copyright, see the "Exceptions > Public Domain" tab. For more information on using another's copyrighted materials in your own work, as well as how to protect your work, see the "For Artists" tab.