See here:
https://github.com/EsotericSoftware/spi ... hment.java
First the region attachment has a width, height, and attachment offset SRT (which is relative to the bone the slot is on). 4 vertices for the image are precalculated in updateOffset(). Next, in updateWorldVertices(), the bone's world translation and 2x2 rotation matrix are applied to the offset vertices to get the vertices in world coordinates.

The rhomboid (diamond) shape for images occurs when the attachment offset rotation is not aligned with the bone and non-uniform scaling is applied (meaning x and y are not scaled the same amount):

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Some game toolkits (like Corona) cannot draw rhomboids/quadrilaterals, they can only draw rectangles. For these toolkits, you can just avoid using non-uniform scaling in Spine, or if you do then make sure that your images are aligned with the bones. An easy way to do this is to click the Parent button, then set the rotation to 0, 90, 180, or 270.