Hi again:
The important requirement which must be met for processing by SDK Resample is that the source files are in the correct GIS cartographic "projection" regardless of whether they are in a GeoTIFF or a BMP file format: SBuilderX will indeed accept either of those (2) file types ...provided that they are "projected" correctly cartographically.
SBuilderX will give the error you reported in the OP above when the source files are NOT in the correct GIS cartographic "projection".
You can "screen" the original GeoTIFF aerial imagery file(s) by using them as a source reference to be read by Ollyau's GeoTiff-to-INF utility to write out a FS SDK Resample *.INF file for each such file, so that you will have Geo-referencing info and other required parameter values for use with the (fixed pixel dimensions) edited imagery...when submitted via a *.INF file output by SBuilderX to FS SDK Resample.
Note as well that Ollyau's GeoTiff-to-INF utility will immediately report when a source reference file being read is NOT in the correct GIS cartographic "projection" required by SDK Resample.
http://www.fsdeveloper.com/forum/resources/geotiff-to-inf.119/
Because the section header in a *.TXT Geo-referencing file that normally accompanies a *.BMP output by SBuilderX has the word "Geographic" in it, technically you could "fool" SBuilderX into accepting the accompanying BMP by editing the Geographic coordinates for that aerial imagery file, but that would most likely result in an incorrect Geo-rectification of the texture image in the wrong GIS cartographic "projection" id the source files are not already projected in Geographic (Lat-Lon) Projection / WGS84 Datum (aka "EPSG:4326").
The proper way to do this would be to open a copy of the original GeoTIFF aerial imagery file(s) in a GIS application such as Global Mapper or QGIS etc.), re-project them into Geographic (Lat-Lon) Projection / WGS84 Datum (aka "EPSG:4326"):
http://spatialreference.org/ref/epsg/wgs-84/
...then export to a new GeoTiff file with an accompanying *.TFW 'world' Geo-referencing file.
Those resulting files can be used in SBuilderX via Add Map > From Disk; at the SBuilderX prompt to "Calibrate" just proceed without performing the calibration, as GeoTiff aerial imagery files in Geographic (Lat-Lon) Projection / WGS84 Datum (aka "EPSG:4326") ...are already correctly calibrated.
I strongly suggest that you double-check your original GeoTIFF aerial imagery file(s with Ollyau's GeoTiff-to-INF utility to verify whether they need to be re-projected in a GIS application.
GaryGB