By default, Lazarus stores the binary image data preceded by the file extension to define the file type. This allows storing multiple image formats at will.
Delphi only allows a single file format (jpg?). Recent Lazarus versions allow this storage format as well and try to autodetect the image type based on magic data in the beginning of the binary data.
See e.g. the sqlite_mushrooms/image_mushrooms example program in your Lazarus directory examples\database\