call post method of dataset after assigning file path:
procedure TForm1.cxImage1PropertiesCustomClick(Sender: TObject);
begin
if OpenPictureDialog1.Execute then
begin
YourDataSet.FieldByName('PicPath').AsString:= OpenPictureDialog1.FileName;
YourDataSet.Post;
cxImage1.Picture.LoadFromFile(OpenPictureDialog1.FileName);
end;
end