I can't add texture into texture paint tool of terrain! Help me.

I read and make same as in Unity3d Documents and some tutorials on Google, but I recieve a messenger (this picture). Please help me, I don't understand this step.

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Texture size is 800x600 for test.

800x600 is not a power of two. 2^2 (or 4) is a power of two, so is 2^3 (8), 2^4 (16), 2^5 (32) etc.