For the REB1200 (ETI-1), use .imp ebooks files from Mobileread's Upload library
ending with _1200.imp
If you find an .imp without such suffix, then it is for the EBW1150 (ETI-2).
When creating .imp ebooks yourself, using eBook Publisher, FizzyWater is right, use the Color (Full) VGA build target i.e. output file type. That's the one for the REB1200.
For more info, see our wiki IMP software
or REB 1200
Also, see the IMP forums, IMP
ebook format and Fictionwise eBookwise