Well, that's why there is the repackage button. So, after you have created the theme, I leave the folder there so that you can edit it as you see fit. You can also add new images in there and if you want, you can add postinst scripts to copy move or whatnot those images, audio, video or whatnot files to any place on the device. So, after you've done your modifications, just click the repackage and it'll make the packaging, but won't extract the images from the template.