No, I don't believe there is any way to change the DPI of .bmp image stored in SAP BDS.
To avoid SAP converting or adjusting bmp image in any way during upload, change it to the necessary output dimensions at 75 (or 100, 150, 200, 300, or 600) DPI, and 8bpp color depth (or 4bpp grayscale or monochrome) outside SAP, then upload using SE78.
The upload functionality (FM SAPSCRIPT_CONVERT_BITMAP_BDS form Bmp_Adjust_Dpi) will attempt to set either 75, 100, 150, 200, 300, or 600 dpi, whichever is the best fit for source DPI. The reason it is done so, I believe, is because SAP documentation claims:
Note that many printer drivers or printers support only the following resolutions: 75 dpi, 100 dpi, 150 dpi, 200 dpi, 300 dpi, 600 dpi.
The conversion due to color depth is performed because OTF BM command can output only 8, 4 or 1 bpp images.
To summarize: no, you can't have 78 dpi image in SAP BDS; no, you can't change uploaded image - only upload again.
cheers
Jānis