Tanks for your feedback!
Tilengine works exclusively with indexed color graphics with 8-bit palettes (up to 256 colors). It uses color indexes inside a palette, not actual RGB values. Transparent color for sprites and tiled backgrounds is always index 0, this is how actual 2D graphics chips used to work and cannot be changed.
It seems that you're exporting 32-bit RGB images from Gimp, not the indexed ones required by Tilengine. They will show ok in Tiled editor, but not in Tilengine. The transparent color attribute in Tiled is only ised by Tiled itself to composite the image, but it has no meaning for Tilengine. Please take a look at this link from the Gimp documentation:
As a working reference I also recommend you to open the assets provided in tilengine samples (/samples/assets folder) in Gimp, because they have the expected format.
Let me know if you have success!