You shouldn't have changed the texture.cfg that came with the paintkit, which looks like this:
[fltsim]
fallback.1=..\..\FSDT_Staircase\texture.01
fallback.2=..\..\FSDT_Baggage_Loader\texture.01
fallback.3=..\..\FSDT_Marshaller\texture.arab_1
the first fallback line refers to the ..\..\FSDT_Staircase\texture.01 folder, which contains all the main textures for the staircase so, by changing it to the location of your logo, caused the missing textures.
Instead, you leave the texture.cfg as it was, inside a new folder named texture.BIMAN you should create under the FSDT_Staircase_USER folder AND put your GSX_logo_4x1.dds file THERE
So, the logo will take precedence of the the files in the fallback folders, but the other textures will be loaded from there.