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.