Virtually all images that appear as part of the UI are built as icons sheets created by the Mod Builder (Burn Icons). These sheets combine multiple PNG images into a single texture so that they can be rendered more efficiently.
Icons from the base data are referenced as Icons_<name>. Icons from mods are referenced as ModIcons_<mod>_<name>. The string must form a valid Action Script 3 identifier, so source asset path delimiters are replaced with underscores, and extensions are dropped.
For example, an icon from the mod 277a61d6d12d4c519ed188822bc60a50, with the relative path abilities\my_ability.png, would be referred to as ModIcons_277a61d6d12d4c519ed188822bc60a50_abilities_my_ability.