Factorio 0.18.1
Gear Girl Character 1.0.3
No other mods.
Failed to load mods: File not found: GirlCharacter/images/level2addon_dead_mask.png

I think you have to set the beta to 0.18.x latest release? No error appears for me. Be sure to set the active version to 1.0.3

I don't get the error either, can still confirm the file doesn't exist though. The above reference is in the data file.

Maybe it matters that i have High quality sprites on?

well there shouldn't be a difference between high quality and low quality, since they both reffer to the high quality sprites instead. So that image doesn't exist yes, but it shouldn't be needed in the first place

I turned on high quality sprites and it worked. Switched back to normal sprites and got the error again.

Maybe I missed removing one of the low quality sprites then, I’ll check and fix it soon then

I had the same issue for the same reason in the 0.17 version, even went as far as to go into the mod's zip to check, the image file's there, mod just don't wanna work if sprites are set to normal.

Maybe I missed removing one of the low quality sprites then, I’ll check and fix it soon then

You did, it's line 465 in data.lua:

      filename = "__GirlCharacter__/images/level2addon_dead_mask.png",

It should be

      filename = "__GirlCharacter__/images/hr-level2addon_dead_mask.png",