Issue Importing a Height Map

I am trying to import height map using the terrain toolbox and it says my resolution is not a power of two. I am using a Photoshop RAW image. I suspect that there is a simple solution to this, but I don’t know how to fix this.

Using photoshop or something else to change your heightmap resolution.
Final resolution have to be 128 x 128 px, 256x256 px, 512x512 px or 1024x1024 px

I hope it will help you =)