I use Photoshop for changing resolution. For lower resolution maps that need more resolution, within Carrara you can select the Interpolate option in the texture map portion of the shader tree. But that will only help a little. It's always best to start with a high resolution texture map and then reduce the resolution, however. If it's a really low resolution map to start with, there is nothing you can do. Actually, I'm waiting for a plug-in that simply makes Carrara do what I'm thinking instead requiring me to do all the work. I'd pay good money for that.