I don't think BMPs and JPGs work as textures. Try making a "grayDark.brickside.png" image in the same folder with the DTS.
I'll try that.Actually it requires .bmp.So, are there any programs that would allow me to change the textures manually on the .dts.
No it doesn't since DTS doesn't save the image format as it is never even defined. You need an image named "grayDark.brickside.bmp.png".