Hello, are you re-elaborating the images exported from Spine after the export?
Spine already optimizes the images on export to use the least space possible. To use this you could change the format on export in Spine to RGBA8888 instead of RGBA4444 for the best possible results (you can find this in the Runtime output section of your screenshot).
You can also get smaller images by choosing the Polygon packing setting instead of Rectangles in the Output section of your screenshot.
If you are determined to re-elaborate the export outside of Spine, you need to turn off Premultiply alpha in the Spine export and at runtime as well, which is likely the cause of the white borders after you re-save the images.
