People like to customise their blogs - and some want to use a favourite photo - not one of the stock Blogger background images. The problem, too frequently, involves file size. I found a background image I really like (or at least I think it would work OK) - but when I resized it to the upload specs detailed in customizing your template, it said the file was too big! Blogger limits background images, as supported by the Template Designer, to 300K.
What Blogger won't tell you