Refer the Odoo 8 stock Module,

create and set image path /your_module/static/src/img/scan.png

Example,

 <div class="oe_right oe_button_box"> 
<button name="do_partial_open_barcode" groups="stock.group_stock_user" type="object" 
class="oe_stock_scan_button" attrs="{'invisible': ['|',('pack_operation_exist', '=', True),('state','not in',('assigned', 'partially_available'))]}">
<img src="/stock/static/src/img/scan.png" class="oe_stock_scan_image oe_stock_scan_image_btn"/>
</button>
</div>