When you want to save your picture field onto the hard disk, you can simply use the WRITE PICTURE FILE command:

For example, the following code will create a .jpg file on disk, next to the structure file.

WRITE PICTURE FILE("Mypicturefile.jpg";[Table 1]Field2;"JPEG")

Please note that this command uses QuickTime conversion routines (version 4 or above recommended) for Mac OS and Windows. If QuickTime is not installed, the command creates a PICT file by default.

For more information on this command, please check the following location:
https://www.4d.com/docs/CMU/CMU00680.HTM