Cannot Create Image with 24-Bit Color Depth

[Marak60]

While trying to create a custom Picture Tube, I got an error message saying that "the image must be 24 bit and have only one raster layer with a transparency. However, I found that I'm unable to create 24-bit images on both Windows 7 with PSP X2 and Windows 8 with PSP X7. There is no such option in the "Color Depth" dropdown on the "New Image" dialog. (only RGB - 8 bits/channel and 16 bits/channel). My monitors are certainly capable and I have DirectX 11 installed. Can anyone help with this?

Thanks,
Mark

[Forriner]

It can be a little confusing:
Generally, when people talk about 24-bit images they mean 8 bits per channel- 8 red, 8 green and 8 blue. Like a jpeg image, for example.
When they talk about 16-bit images they mean 16 bits per channel.

I don't use picture tubes myself, but I tested something.
I created a new image, color depth 16 bits/channel RGB
I tried File/Export/Picture tube... and got the same message: 24 bits, and transparency.
I Image/Decrease color depth... to 8 bits per channel RGB

Disregard thew red: LeviFiction's post below explains it.
I tried File/Export/Picture tube... and got the same message: 24 bits, and transparency.
I used the eraser tool to erase a bit of the image so Raster 1 showed some checkerboard transparency.
I tried to File/Export/Picture tube... and got a dialog to save it. I stopped there but I learned:

So: 8 bits/channel RGB -and-
just one single raster layer needed to save a tube.

You can create it at any color depth, just Image/Increase or Image/Decrease color depth to RGB 8bits per channel

[LeviFiction]

No parts need to be transparent. The layer simply needs to be a raster layer and not a background layer. So if you start with an image that has a "Background" layer just promote it to a raster and it should work.

[Forriner]

No parts need to be transparent. The layer simply needs to be a raster layer and not a background layer. So if you start with an image that has a "Background" layer just promote it to a raster and it should work.

Oops yes you're right. By erasing a piece, PSP automatically converted the Background to Raster. Getting late, time for bed

[Marak60]

Thanks guys for both the education and the solution. Promoting the background layer did the trick. And now I know that 8 bits per channel *is* 24-bit color depth!.