I also have a ton of Orthos from my X-Plane installation, so looking around and collecting info from other posts this is what I did:
Copy your XP Orthos into the Ortho4XP-FSXP3D "Orthophotos" folder
Google "reduce_imagery.py" it belongs to Oscar, the Original author of the tool and download it.
Have Python 3.7 installed (not sure if it´s compatible with other versions)
Edit the file and replace "jpg" with "bmp" in line 33
Paste the file in the Ortho FSX folder where the BI_16 folder is located (if used Bing and ZL16, if not replace it with the proper string)
open a cmd window in that folder and type "py reduce_imagery.py BI_16 16" it will convert all the jpg files into (new) bmp files inside that folder
Run Ortho4XP-FSXP3D, it will skip the Ortho Download automatically since the files are already there
In my machine it takes aprox 10 minutes to generate an FSX tile based on existing BMPs
Regards