If those stroke are on their own layer, you could try adding a blur to them.
Pls...guide me to design backgrounds (like abstract wave backgrounds)..
Do you have a tablet?
Google up some cloud images to use a basis. Something with lots of blue sky. Use the search options to find higher resolutions, or just upres them in Photoshop. Copy the BG layer a few times so you can make multiple attempts. Try Radial Blur, Liquify. Or use the pen tool to make a nice Bézier waveform through the cloud, and stroke the path with the Smudge tool.
Search for Photoshop Smoke brushes. There's lots of them, and they might make a better start point than clouds.
Try a Levels layer, and drag the black output slider towards the centre to give it that muted colour look.
Hmmm... No tablet? No problem. Use the pen tool to make a nice S shape with pointed ends and a fat middle section and fill with white. Use the above methods to mess it up.
Go get your camera, and take pictures of moving objects against plain backgrounds using long shutter speeds to give motion blur. Or zoom while taking the picture, or rotaing or moving the camera any way you can think while taking a picture. Use long shytter speed and run forwards while taking a picture.
I could go on all night.
If you resample your original background image down to a smaller size you might consider the following options (or even combine them):
1. Before making the original image smaller, go to Image - Mode and check 16 Bits/Channel. This will help to retain more gradients. For comaptibity set this back to 8 Bits/Channel before printing.
2. Use a Blur filter for the background (for instance Gaussian blur, set to a radius of 1.0).
3. When changing the Image Size you might want to use the Resample mode of "Bicubic (smooth gradients)".
Perhaps you can place an image that shows the "strokes in between"?