If you have been to my personal blog lately, you might have seen an image split into 3 sections. One above the banner (uploaded at photobucket), another is the banner itself, and one as the image at the very top of the sidebar.
The blog layout plays a part of it. When changing the layout, you may need to change it again or edit the html of it. If you need to resize, use paint and not the checkbox that appears when uploading the image. If you know how to edit the HTML of the layout this should be easy, though finding the specific code can be a pain.
For the image at the very top, you need to upload it at a 3rd party site and then put the following code just next to the page title at the very top of the codes like this: < src="(image url)"> (the quotation marks (") and slash (/) are required as omitting them would result in an error when saving it).
Having the same background colour as the image is optional but can make it look better. Do note that when adjusting image for the bottom part, pay close attention to the margins. Make sure the with is fixed (in px) and not in percentages (%) or no border. The width of the sidebar doesn't matter but it's recommended that the width of the image uploaded there is the same as the header image.