It would take a modification of the programming to stretch the wallpaper to fit the full size of the screen, and the screen size varies by device and orientation. Perhaps disable it on mobile devices with a style sheet.
In general, the contrast or visibility of a wallpaper can be modified in the style sheet as well, sort of a fading if the image by giving it a percentage of transparency. These terms I am using are terms that HTML programmers use when creating the style sheet (CSS) that a web page uses to display the various tags on a page.
Let me throw out my two cents on decorating a profile, if I may. According to our standards, colors, shapes, bold, underline and italic text should not be used if the editor must use HTML tags to produce the effect. I have seen so many pages where the decorations take away from the purpose of what we are trying to do here. Images that flash or change, text that scrolls like a marque are examples. Lines between sections and fancy scrolls are another. Some folks like them, but they do little to make the page readable and nothing to help with accuracy, one of our goals. Here is an example of what I am referring to. Note the various style used on the page.