Depending on your design sense and Start screen tile layout, one new.1 feature may really grab you: The ability to use a photo or other image as a Start screen background. Oddly, this feature doesn't work as it does in Windows: With Windows Phone, the background comes through on the tiles, not on the space between. And the effect can be quite interesting.
For example, consider the following Windows Phone 8.1 Start screen. It's fairly typical, bland even, with a single accent color on each non-customized tile.
But with the new Start Background feature, you can change this. All of the non-customized (i.e. opaque) tiles can now be adorned with a background image. So you can create some surprisingly different-looking effects, depending on which image you choose. Here's the same Start screen with three different backgrounds.
Best of all, there's a neat parallax scrolling effect too. I can't display this effect in a screenshot, so check out this video I made recently:
Here's how you can customize your Windows Phone 8.1 Start background.
First, navigate to Settings, Start + Theme.
Now, tap the Choose Photo button to find a photo or other image. In the Picture Chooser that appears, you can look in your camera roll, a Backgrounds folder that's built into Windows Phone, or other online locations like Facebook and OneDrive. I'll use the Backgrounds folder for this example.
Select the image you want and crop when prompted. You'll be returned to Start + Theme settings and will see your selection.
Now, navigate back to the Start screen to see whether this is the effect you wanted. You may need to play with tiles sizing and placement to get it just right.