There are two ways to use custom decorations (textures):
Override db content Edit
You can override any image in the LDD database with the technique of overriding db content. Just place your custom PNG image there in place of an existing one and it will be picked up by Blueprint and Bluerender (and LDD too).
This method will override the database items for all images you render.
Decoration substitution Edit
This method uses the scene file and can therefore be customized differently for each LXF file you render (see using custom scene files). Because it uses the scene file, this approach only works with Bluerender.
For example, to replace the LDD decoration with id 59879, you can add a line in your Bluerender scene file in the following format:
##CHANGEDECOR 59879 c:\path\to\custom\mydecor.png