Batch Download Pictures From Website

well i am downloading images from a php gallery that contain 3 sizes for the images meduim small and large linked in the same page but each is in a unique folder /img_s/ /img_m/ /img_l/ however these folders aren't in the main link the link is follows http://website/database/showimg.php?Id=10&size ...

i have multiple tasks in one project
i want to start the tasks one after the other automatically
not together but one after one ??

How can i do this