I don't post here often. If it's in the wrong place, please feel free to move it. I'm just putting this here, because when I did a google search I came up with nothing.
First of all, I use STEP as a base and have for several years now. Please don't come back on me telling me I wouldn't have had this problem if I'd followed the guide exactly. When I deviate, I do it with full knowledge that I might be creating my own problems.
This one, I think is too easy to solve not to add it into the guide, though.
If a player is experiencing missing fire textures using DSI, especially with two spell impacts option, they need to verify the load order. DeadlySpellImpacts - Two Fire.esp MUST load before DeadlySpellImpacts.esp.
LOOT consistently puts these two mods in the wrong order. User meta data should be added to load DeadlySpellImpacts.esp AFTER DeadlySpellImpacts - Two Fire.esp. If anyone wants to report this to the mod author or the LOOT rules team, feel free. I'm going back to playing.