1- Download Sony Image Viewer from 'Programs' page.

2- Download the suitable firmware file for your phone from 'Firmwares' page.

3- Prepare the wallpaper you want. It should be in 4-grayscale BMP format, 96x77 in size. You can download the template or a ready wallpaper from 'Wallpapers' page.

4- Run Sony Image Viewer and load the firmware file into it.

5- Find the right start address of the wallpaper. A list of addresses for different phones and languages can be found in 'Information' page.

6- Select 4P/B compression format and click 'Draw'. You can see the wallpaper now.

7- Adjust the size of the wallpaper to 96x77 with the two buttons in the bottom-left corner.

8- Click 'Load BMP' and select the wallpaper you prepared.

9- Check 'Turn On Edit Mode', then click 'Save Changes'.

10- Flash your phone with the modified firmware file.