To start, locate the image that you want to replace the boot screen. It should "weigh" no more than 256KB and have the extension *.jpg, or *.jpeg. The resolution should match the resolution of your monitor. Rename the image to backgroundDefault.
To learn the format and image size, open the section in which it is located. Click the right mouse button in the space section and from the menu select View > table. How to display files in the directory will change next to each graphic file will appear several columns, including "Type" and "Size", which specified the parameters you need.
To find out the resolution of the file, click right mouse button and from the appeared menu choose "Properties" and then tab "Detail". Here is a list of file parameters, grouped. Find the group "Image", and in it the paragraph "Dimensions". To the right of it indicates the picture resolution.
Press the key combination Win+R in the window that appears in the input line enter regedit and press Enter on the keyboard. Then you may receive a window requesting your permission to make changes to your computer. Click "Yes". Opens the registry editor.
Open the registry editor directory HKEY_LOCAL_MACHINE > SOFTWARE > Microsoft > Windows > CurrentVersion > Authentication > LogonUI > Background. If you did everything correctly, in the right part of the window you will see the settings related to the loading screen.
Locate the setting OEMBackground. If it is missing, create it. Right-click the white space in the right part of the program and in the opened menu, click "New > DWORD (32-bit)". Name this option OEMBackground, then open it and set the value to "1".
Copy the previously prepared image in the directory C:Windows\System32\oobe\info\backgrounds. If the folders info and backgrounds do not exist, create them. Restart the computer so the changes take effect.