OK just checked, RGB - 8bits/channel is the same as Paints 24bit.
However, I think I've worked out not why it's not working, but how to get it working for me. For some reason it didn't like me taking the image I was using and converting it to 24bit BMP, but when I copied the image and pasted it onto a new 24bit BMP it worked fine, must have been some channels in the original one that didn't convert properly or something, odd that.
Primary Desktop:
i7 7700,k NV1070 8GB, 16GB 3200mhz memory, 1x 2TB Hybrid, Win10.
Secondary Desktop:
i5 4760k, NV960 2GB, 16GB 2333mhz memory, 1x 2TB Hybrid, Win10.
Laptop:
i3, Intel 4000 series graphics, 6GB memory, 1x 500gb HDD, Win8.1.