Bud, when making a theme you must try to make some part as small as possible. Because someone else may not have such a big screen or use such big images. example, if you have 10 items in your startmenu then maybe your startmenu height will be 500px(example) and maybe someone else is using 5 items then his startmenu height will be 250px. if your startmenu image is 100px(the height i mean), and it is stretched/tiled right then no one should have a problem. the image would automaticaly stretch/tile to 250px or 500px. BUT if the startmenu image was 500px then the person with 5 items would get a cutted image, because the image would then have to resize from his original size(500px) to a smaller size (250px). So try to make the images small.
If you have a high detailed image and you want that it should come completely maybe 200px width and the default size of that part is 100 px, then you can change the margin using CONTENT MARGINS. this puts a margin between your content (maybe text) and you bg-image.
About saving your images, i use standard PNG, not-interlaced. I haven't used PNG for web and devices because i think it slightly changes to colors to make it 'websave' (i am not so sure about it bud). Websave has to do colors, a whole different story.
If you want to know more, just ask then bud.