You can create these fractal images in different dimensions and colors and with different transparency levels. And once you create any image, take the screen shot [PrtScr] and paste it in MS Paint, to save it as an image. And finally you can use them as background images in your blogs.
If you want to create images like those shown above (of course within minutes and without touching Photoshop), then visit this site: Guilloche
And also you can visit their Galleries, for some pre-made fantastic images: