1) It was done on the pixel-by-pixel level in Adobe Photoshop CS5
I did the same thing and had the same artifacts that you're seeing, it's quite hard to chop up an image by hand.
2) No. I'll research this. If its simple to explain, feel free to explain it on this topic
Here's some cut-and-paste pseudo-code code from texture loader class used by my skybox builder (using LWJGL):
final int type = GL_TEXTURE_CUBE_MAP;
allocate texture cube-map
upload the 6 texture images
final int wrap = GL12.GL_CLAMP_TO_EDGE
glTexParameteri( type, GL_TEXTURE_WRAP_R, wrap );
glTexParameteri( type, GL_TEXTURE_WRAP_T, wrap );
glTexParameteri( type, GL_TEXTURE_WRAP_S, wrap );
apply min/mag filters
3) Yes they're on every edge, it's probably the wrapping policy that will fix it.
How would I go about separating the skybox image via an algorithm? PNG decoder?
Dunno, anyone else have any advice on how to separate the 6 texture images?