Icons are textures used for many of the in-game GUI items.
Sizes and Format
They generally are TGA format to increase clarity - although DDS can of course be used (which may be recommended if doing high resolution icons that are scaled up and down with GUI scaling NWN:EE has).
They can be increased in size past the default sizes Bioware originally intended but this comes at the cost of them not loading in the toolset properly. To work around this you could work on a toolset version of hakpack files or override content with original sized icons, and switch to higher resolution ones for distributing to players.
Icons of course have transparency - especially in the inventory - which is where there is an absence of pixels in the TGA/DDS file.
The main thing if changing the size is the ratio should stay the same else stretching would presumably occur - or possibly the texture won't load - so doubling the size is easiest for a HD upgrade, or 4x is also possible if you do AI upscaling.
|Icon Type||Example Usage and icon prefix||Default Game Resolution||Double Resolution Size||Notes|
GUI icons (used in action bar/radial/quickbar) - ir
Spell scrolls - iss
Spell icons (used in levelup/spellbook/radial/quickbar) - is
Feat icons (used in levelup/radial/quickbar) - ife
Skill icons (used in levelup/radial/quickbar) - isk
Domain icons (used in char sheet/leveup) - id
Effect icons (used in char sheet/sometimes top right) - ief
Inventory 1x1 icons (eg; Gems) - i (but usually itt)
DM action icons (DM mode) - dm_
|1x2 Inventory||Inventory icons - bracers etc. - i (but usually itt)||32x64||64x128|
Inventory icons - torches etc. - i (but usually itt)
Note the bottom 32 pixels are not used by the game
|2x1 Inventory||Inventory icons - Small weapons, arrows, etc. - i (but usually itt)||64x32||128x64|
|2x2 Inventory||Inventory icons - boxes, traps etc. - i (but usually itt)||64x64||128x128|
Inventory icons - cloaks, armor etc. - i (but usually itt)
|64x128||128x256||Note the bottom 32 pixels are not used by the game so should be left blank|
See Hardcoded Icons for a basic list, these are loaded by name by the engine, not from a 2da file.
Non Hardcoded Icons
These are referenced by 2da directly or indirectly. Many of these are pretty much only there for the TLK reference and icon.
|2da file||Bioware Default Prefixes||Icon Type|
|actions.2da||ir_||A subset of actions - not all of them are here naturally, but most are. The icons appear in the radials, and top left "action queue" as appropriate.|
|classes.2da||ir_||Class icons are shown generally in levelup screens only. Note creature classes just default to ir_wizard (or ir_cleric for oozes) because it doesn't really matter (note for Ooze, most of the cleric line was copied)|
iw (default weapon icon)
iit_ (default item icon)
Shown in all sorts of places. The "Defaault" ones shouldn't usually show up.
Domain icons as show in the level up and character sheet.
Effect Icons such as "ief_damresist" for damage resistance, on the character sheet, examine panel and top right of the players screen for effects on self.
Helpful tip; different immunities, or damage type immunities can be specified exactly here (eg; Immunity to Critical Hits can have a different icon from all the other immunities)
is_ (usual spells)
ir_ (several generic monster abilities/activate item)
|Spell icons, used in levelup, spell books, action radials, spells cast from non-scrolls etc.|
Scroll icons - the icon that replaces the blank scroll directly (ie it has the scroll background etc.). If the spell is being cast from another item type it'll go into a radial and look for the usual spell icon (see spells.2da)