Hello Amarnadh,
In ADD_BUTTON_ROW_ELEMENT method, we have a parameter called IS_SPECIAL_PROPERTIES. This structure contains the 'IMAGE_SOURCE' as one of the property.
Fill this structure with this property and pass SPECIAL PROPERTIES parameter while calling ADD_BUTTON_ROW_ELEMENT method.
Sample Code:
data: ls_special_properties type FPMGB_S_SPECIAL_PROPERTIES.
ls_special_properties-image_source = '~Icon/Search'.
io_layout_config->add_button_row_element(
EXPORTING
iv_display_type = 'BT' " Action Display Type for Toolbar
iv_index = 1
iv_text = 'Button Text'
is_special_properties = ls_special_properties " Special Properties for UI Elements
it_element_action = lt_action " Actions in Configuration
iv_br_id = lv_br_id ). " FPM: Element ID
Hope it will helps you. 
Regards,
Naga