In fact I did not use any API functions. It is done by couple lines of simple code, that you can see below. This code I used to use in my App FRX2ANY.ZIP
in order to extract BMP image from Gen field into file. If you will find something that can be improved, let me know.
Best wishes

*****************************************************************************
LPARAMETERS tcGenFldName

LOCAL lnInHandle
LOCAL lnOutHandle
LOCAL lnFileLen

LOCAL lcTmpFile
LOCAL lcBmpFile
LOCAL lcString

LOCAL loResult
LOCAL lcOldComp

lcTmpFile = ADDBS(SYS(2023)) + SYS(2015)
lcBmpFile = ADDBS(THIS.cReportPath) + SYS(2015) + '.BMP'

COPY TO (lcTmpFile) FIELDS (tcGenFldName) NEXT 1

lnInHandle = FOPEN(lcTmpFile + '.FPT')
lnOutHandle = FCREATE(lcBmpFile)

IF lnInHandle >0 AND lnOutHandle > 0

lnFileLen = FSEEK(lnInHandle, 0, 2)
= FSEEK(lnInHandle, 0)

lcString = FREAD(lnInHandle, lnFileLen)
lcString = RIGHT(lcString, LEN(lcString) - 599)
=FWRITE(lnOutHandle, lcString)
ELSE
lcBmpFile = ''
ENDIF

=FCLOSE(lnInHandle)
=FCLOSE(lnOutHandle)

IF FILE(lcTmpFile + '.FPT')
DELETE FILE (lcTmpFile + '.*')
ENDIF

RETURN lcBmpFile
******************************************************************************