Hi,
I've tried this with gimp and PNG format in LibreCAD 2.0.0rc1 on Linux, for me it looks good.
With gimp I made an image with transparent portions and reduced the opacity to 20%. Then saved it in PNG format with transparency.
When I add this image with the Draw->Insert image function, it is added in the background and also printed with the opacity I applied in gimp.
Maybe you can try to use the Save for Web feature of Photoshop or changing the brightness of the image.
Good luck
Armin
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