Reducing app weight & global background

Hi! I am making a 62 pages ebook about animals & plants; 1024 x 768 size. Each page will have between 4 - 8 illustrations of about 400 x 400 (they will appear one on top of another, as layers..) I compose each page in Photoshop and the import it to interact builder. Given that the background will be the same for every page; I thought on importing it just once and making it global, to reduce app weight. But I have some doubts:

1- is it possible to convert a background element, imported within a psd file; into a global element (global background not clickable)

2- Although I have deleted all the graphics in the pages I had made, to leave the pages blank again, the app size didn't changed in the info pannel... I cleared cache also, but it marks still 38,72 Mb... how can I effectively delete elements in order to reduce app weight?

3- Does interactbuilder makes any compression on images imported? My psd source files are 5 -10 Mb big each; so for a book of 60 pages it makes 300 - 600 mb size app... isn't that too much? how could I reduce the app weight? (I am working with 1024 x 768 px; sRGB 8 bytes psd 72 ppi psd files...)

Thanks a lot. I know there're many questions, sorry for that : /