Black Background Color after conversion PNG images to BMP

We are getting the xstring legacy system to an image of type PNG with transparent background and converted into BMP, the image background is black.

**Converter de PNG para BMP
CREATE OBJECT l_igs_imgconv
EXPORTING
destination = 'IGS_RFC_DEST'.

li_input_length = XSTRLEN( w_xstring ).
CALL FUNCTION 'RSFO_XSTRING_TO_MIME'
EXPORTING
c_xstring = w_xstring
i_length = li_input_length
TABLES
c_t_mime = ti_blob_in.

l_blog_size = li_input_length.
CALL METHOD l_igs_imgconv->set_image
EXPORTING
blob = ti_blob_in
blob_size = l_blog_size.

l_igs_imgconv->input = 'image/png'.
l_igs_imgconv->output = 'image/jpeg'.
*l_igs_imgconv->output = 'image/bmp'.
l_igs_imgconv->width = 640.
l_igs_imgconv->height = 480.

CALL METHOD l_igs_imgconv->execute
EXCEPTIONS
OTHERS = 1.

IF sy-subrc IS INITIAL.

CALL METHOD l_igs_imgconv->get_image
IMPORTING
blob = l_img_blob
blob_size = l_img_size
blob_type = l_img_type.

DATA: lt_table TYPE TABLE OF lxe_xtab,
wa_table TYPE lxe_xtab.
LOOP AT l_img_blob INTO wa_img_blob.
wa_table-line = sy-tabix.
wa_table-bytes = 255.
wa_table-text = wa_img_blob-line.
APPEND wa_table TO lt_table.
CLEAR wa_table.
ENDLOOP.

CALL FUNCTION 'LXE_COMMON_TABLE_TO_XSTRING'
IMPORTING
ex_xstring = w_xstring
TABLES
in_tab = lt_table.

ELSE.

CALL METHOD l_igs_imgconv->get_error
IMPORTING
number = l_err_code
MESSAGE = l_err_text.
ENDIF.