Is posting a link in a project to a reference image against the guidelines?

Absolutely! Giving credit to your inspiration is encouraged.

However, please make sure that your reference image doesn't contain anything inappropriate or offensive. If you aren't sure whether your image will pass the guidelines, it's always a good idea to just not post it. 

Please consult the Khan Academy guidelines and use your best discernment before posting something that others will be able to see.

I hope that helps!

Thanks! It really helps :)

By "so long as the image does not violate the Khan Academy guidelines", I meant that the image should not contain anything inappropriate, offensive, or anything else that could be considered against the guidelines.