Windows 7 pictures

Default user pictures are stored in C:\ProgramData\Microsoft\User Account Pictures. The format is a bitmap 128x128. Default login screen backgrounds are stored in C:\Windows\System32\oobe\background.bmp and C:\Windows\SysWOW64\oobe\background.bmp. The format is bitmap 1024x768. You can change the default background if you create C:\Windows\System32\oobe\info\backgrounds\backgroundDefault.jpg. 96% quality (cf. gimp) is not supported but 85% quality is OK. Then it is possible to keep the same background for the login screen and your desktop. You might need to set HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background\OEMBackground value to 1 (dword).