The files in the asset pack (plus a few that aren't) are part of the base game, and contained in grimrock.dat
An item
* must have a gfxIndex
* can optionally have a gfxAtlas
If an item has no gfxAtlas, then the default gfxAtlas is used.
The default gfxAtlas consists of these files, chained together:
* assets/textures/gui/items.dds
* assets/textures/gui/items2.dds
The indices in a gfxAtlas are calculated automatically.
Index 0 is the leftmost icon in the top row.
Index 1 is the icon directly to the right of icon 0.
When you reach the rightmost icon in a row, increasing the index by 1 goes to the leftmost icon in the next row.
The item icons are 75x75 pixels each, but the width and height of a DDS file must both be a power of 2.
(not necessarily the same power of 2, AFAIK)
That's why items.dds is 1024x1024, even though only 975x975 are actually used.
If an item has a gfxAtlas:
See Isaac's explanation.
http://www.grimrock.net/modding_log1/cr ... om-assets/
has a section on "Using Custom Graphics"