cubemap background problem!

How do you create a cubemap when you’ve got your six images without using photoshop?

Trying to do this and failing without photoshop:

I’ve also tried to edit the existing T_Sky_blue texture to include objects such as buildings, but can identify which bit of the texture corresponds to which area in the game, so my buildings are going on the ground or in the sky.

The skysphere >> Sky material >> “Material Instance Dynamic” can’t be disabled either?

Need some help urgently!