Those are also BAMs. The in-game ones for the description sceeen all start with C, I believe. For example CCLCK20.bam is the description screen image where ICLCK20.bam is the inventory image for and item called CLCK20 (Cloak of the Shield). You can review all these in NI. To make my own description pic for a custom BAM, I save the palette off a .gif copy of an in-game description BAM in Photoshop using the "Save Color Table." I then convert my new inventory icon to that palette with "Load Color Table." Most graphics programs probably have a way of doing the same thing.
I usually make items in DLTCEP, where you simply type in the bam names in the "Icon & Item atributes" tab. In NI, the description BAM is set at the "Carriedicon" for the item.