Skyboxes

I was making a custom skybox for my level. but theres a problem the skybox looks horrible... you can see the seams easily. Just wondering if there was a tool to stitch the pictures together.

Unfortunately, as far as I know there is not a tool to stitch the pictures together. If you're using Terragen to render the skybox, make sure the images are rendered with the camera in the same position for every picture, with a field of view of 90 degrees.

Moved to Custom Graphics

What tool did you use to create the skybox? Generally you should use a tool like Terrage (or blender if you're insane) and render 6 faces.

If you want to stitch them with free software, try Image Composite Editor if you have a Windows computer.
I'm not sure if it will work very well though, because of the big seams and the two identical sides with one being mirrored. And also because that program can be really dumb sometimes.

Im using terragen

What are the settings for your camera? Is the Field of View exactly 90 degrees? If not, the images won't match up when you render them.
Also, it looks like you're rendering one image and flipping it to make the "walls" of the skybox. What you should do is render an image with the camera's yaw at 0 degrees, render a second at 90 degrees, a third at 180 degrees, and the final one at 270 degrees (aka -90 degrees).

Yes I made it exactly 90 degrees and then the next render exactly 180 and etc.

Is your camera zoom (can't remember what it's actually called) set at exactly 1? If it's not at 1 the images won't line up.