G0ldiel
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Post an improved texture here (army, inventory, or abilities) and a screenshot of what's wrong, so I can take a look at it on my end.
In the paths Textures/HeroScreen2/Inventory/Cast and Textures/HeroScreen3/Inventory/Cast (I double-checked with the original data folder; the paths to this folder and the final file are actually two – I don't know why), the Normal.dds file has been modified – the resolution has been doubled (from 69x69 to 138x138). In the Normal.xdb file in both paths, this change has been taken into account. This file is responsible for the image of the spellbook icon in the hero viewing menu.
There are related files at the paths UI\HeroScreen2\Inventory\Cast\Normal.xdb and UI\HeroScreen3\Inventory\Cast\Normal.xdb, but I don't see anything interesting or related to the resolution that needs to be changed.