Windows 7 allows you to set your own image for the welcome screen which appears when you start your computer. The setting is well hidden, so you'd be forgiven for not realising it existed.
First, you need to change a Registry setting. This is easily accomplished: in the Start menu search box, type regedit and press Enter. Navigate using the tree structure to:
Double-click on OEMBackground and set the value to 1 (typically it will be set to 0 unless you happen to own a laptop or PC which has already got a customised logon screen - Sony, HP and other manufacturers use their own images on certain models).
If you don't see OEMBackground this in the right-hand pane, simply right-click in an empty area and create a new 32-bit DWORD with exactly that name, and set its value to 1.
Finally, place your JPG image (which must be less than 256KB, but should match your monitor's resolution) in the following folder:
C:\Windows\System32\oobe\info\backgrounds (you'll have to create the info and backgrounds folders yourself)
Bear in mind that changing your Windows theme will reset the Registry value you changed, so you'll need to set it back to 1 if you do change the theme.
If your photo renders your username unreadable, you can make a second Registry tweak to add (or remove) shadows from the text and buttons. To do this, launch the Registry Editor as before and navigate to
Create a new DWORD (if it doesn't already exist) called ButtonSet.
Set the value to 0 for a light shadow, 1 for a dark shadow or 2 for no shadow at all.