I have seen that many stock-providers allow you to use your stock as long as credit is given. When people use my stock, they usually pay attention to this commonly accepted rule and they give me proper credit. However, my stock rules are DIFFERENT. I don't need credit, I just want you to follow the licencing rules - which is the Creative Commons Attribution-Noncommercial-Share Alike 3.0 License.
That "Share-alike" means: "If you alter, transform, or build upon this work, you may distribute the resulting work only under the same or similar license to this one." So, if you want to build upon my stock, you need to licence such picture under the