Hi,
You can use class cl_wd_runtime_services method attach_file_to_response.
In the onAction of button get the image data and pass to the method:
cl_wd_runtime_services=>attach_file_to_response(
i_filename = <file_name>
i_content = <file_data>
i_mime_type = '<MIME>' )
Or you can simple use Filedownload UI. For reference check this wiki: http://wiki.sdn.sap.com/wiki/display/WDABAP/Upload+and+Download+files+in+Webdynpro+ABAP
Hope this helps u.,
Regards,
Kiran