another thing....

I use GIMP image editor, but when I export to my desktop in a R5G6B5 .bmp format, OHRRPGCE won't load it.
I use Unbuntu, if that makes a difference

you have to use 256 format to upload to OHR custom

May I have a walkthrough please?

When you actually save the image as a .bmp in GIMP, go to the advanced options menu that shows up before the image is actually saved and choose 16-bit. It seems that there are three, but just choose the first one on the list.
If that's what you've been doing, just use something like Irfanview to force the palette down to 256 colors.
[Personally I use GraphicsGale so I don't have to worry about palette issues all that much, but...]

In Gimp, go to Image->Mode->Indexed

then choose the number of colors, 256 for backdrops and maptild sets, or 16 for any kind of sprite.

Then when you save as BMP it will use the correct bit depth automatically.