If you also release them for public use before the contest starts.
You can use content you make yourself without releasing it. However, if it's not your own, then it has to be public. This is a rule I plan on changing in the future (so that all custom content has to be public), but I don't want to change the rules after already announcing the contest.
I am always told that you can't use content you made before the contest without releasing it before the constest starts. There's too many edge cases between "working on your map before the contest starts" and "working on private custom content before the contest starts".
Ok, so I can create custom textures before contest, only if I release them to public, right?
Pretty much.