Because the Dell Streak handles wallpaper differently than other Android devices, letting you set a different background for each home screen, it also uses a different resolution than what you might expect.
The wallpaper resolution of a "regular" Android device with a 480 x 800 screen is 960 x 800 (the doubled width allows the image to span across all of the home screens). But if you try to set a 960 x 800 image as wallpaper on the Streak, you'll be forced to crop it. The crop box can be enlarged to grab more of the image, but it will never be able to cover the whole thing. And if you turn to Google for help and download a third-party app like Wallpaper Set & Save, which many people recommend, you'll discover that it creates an aspect ratio issue that makes the image look worse.
What you need to do instead is use images with a resolution of 800 x 444. Anything bigger than this will need to be cropped, no matter which app you use to set it as wallpaper.
Why not 800 x 480 to match the resolution of the Streak's screen?
The top bar. Although it contains a lot of useful information, it also uses up 36 pixels. The bar is a permanent fixture on the home screen, so you need to accommodate its presence if you don't want to crop your wallpaper.
You won't be able to find many ready-made wallpapers for 800 x 444 screens anywhere, so you'll need to either create them from scratch or modify existing wallpapers made for other devices. If you create your own wallpaper and would like to share it, please show it off in the Streak Smart forum.
The downside to the latter method is that you can't ever get the full image; you always have to crop. The wallpaper you see in the screenshot above, for example, is one that was made for WVGA Android devices that use 960 x 800 wallpapers. This is the original image (resized below):
As you can see, I had to decide which parts of the image to crop for the Streak. It's still a great wallpaper, but I would've liked to be able to use the entire image.
Once you have an 800 x 444 image you want to use, just load it onto your Streak's microSD card by connecting the device to your computer, emailing it to yourself, and so on.
If you use the stock Photos & Videos app (not sure why this isn't called Gallery) to set the image as wallpaper (Menu -> Set as -> Set as wallpaper), you'll still be presented with the crop box. This time you'll actually be able to drag the corners to cover the entire image, though, so nothing will be lost.
If you use an app like Wallpaper Set & Save, on the other hand, the image will be set as wallpaper automatically.