this is Simwings Munich for example. I can choose logos from both drop-down lists but I can't upload own logos...
Ok, now I understand.
Of course you can't choose a custom texture, you are using P3D V5, that's entirely normal. Custom textures requires DirectX 11, which is not supported in P3D V5 and we have no plans to add DirectX 12 support, with reasons that has been discussed many times on the forum.
Render To Texture with DirectX 11 is replaced in P3D V5 by standard multi-liveries, like in FSX. Which means you CAN choose an operator from the included ones, but you CANNOT upload your own texture.
When I said I tried your texture and it worked, I meant in P3D 4.5!! I assumed you knew those won't work in V5, so I tried your texture with 4.5, to be sure there wasn't any issues with it ( the size is not a problem, since if it's bigger than 1024x256 it will be automatically resized to 1024x256 ) and they worked.