First, choose an image that you would like to convert to a Vita wallpaper and select it in the "Choose image" panel.
After that, select the number of home screens in the "Choose number of screens" panel across which the wallpaper should be drawn.
Next, you can choose one of the scaling options in the "Choose scaling" panel to adjust the way the image will be stretched or scaled across the wallpaper.

Available scaling options:

• Tile will repeat the image in its original size across the size of the background.
• Zoom completely fills the available space by enlarging or reducing the original image to fit the whole wallpaper (might cut off areas of the image).
• Center puts the image in its original size right at the center of the wallpaper.
• Scale enlarges or reduces the original image size to always be completely visible with no cutoff areas.
• Manual allows for manual positioning and resizing of the original image onto the wallpaper by method of drag and drop.

Finally, hit BAM to submit the image and view your backgrounds!

Now all that's left is to download the background(s) to your Vita and set them as home screen background.
If you used this tool in the Vita browser, downloaded images are in the photo album.
After you transferred the backgrounds to the Vita, tap-and-hold a finger on your Vita home screen until the edit mode activates. Then tap the little wallpaper icon in the lower-right corner and navigate to the downloaded image and select it. Repeat for the other backgrounds and home screens if there are any.

All done!

Creating a lockscreen follows the same steps as creating a background, with following differences:

Instead of selecting the number of home screens you have, you only need to select 1 screen in the "Choose number of screens" panel.
Also, for a nice unzip-effect when unlocking your Vita, try enabling the Zippers under "ADD MORE ZIPPERS".

After you downloaded the resulting lockscreen onto your Vita, set it via the Photo Album application by navigating to the image, opening it and tapping "Use as Start Screen" in the "(...)"-bubble-menu.

This site supports images in JPG, PNG, and GIF formats smaller than 2048x2048 pixels and bigger than 256x256 pixels.
The resulting backround files are in PNG format at 960x544 pixel resolution (exactly the size of the PS Vita screen).

Sadly, at this time, this tool does not (and neither does the PS Vita) support animated wallpaper backgrounds.

Create your own wallpapers for use on the new Playstation Vita.
Just choose a picture from your computer, select how you'd like
to scale it and the number of home screens you have, and BAM!
You now got your own wallpaper!

It also supports manually scaling and moving the image on the
background via drag and drop for maximum control.
The guides are shaped like a Vita home screen and will help
you align the most important part of your background
correctly for a neat wallpaper.

You can easily create spiffy lockscreens with the cool
zipper-look as well. Just choose the "Add more Zippers" option
and you're good to go!

http://vitawallpapers.tumblr.com/ for the awesome zipper.

Standards: HTML5, CSS3
Components: Modernizr, jQuery
Software: HTML5 Boilerplate
Google App Engine