If you're not a fan of the glow effect, I suggest you ignore this thread ~_~"
I'm going to be using Adobe Photoshop CS2. I'm pretty sure that my methods will also work in some of the older versions.
As you may know, .jpg files are commonly used because of its small size. However, whether it be desktop wallpapers or forum banners/avatars, .jpg isn't the way to go.
So, anyways, I'll start off with a .jpg picture of Nodoka falling on Negi.
If you're keen, you would immediately notice the blocks of colors clustered together. That's the nature of .jpg files.
Anyways, I copy and paste this image into Photoshop.
The first thing you do is create a duplicate layer of this image. The shortcut for this is
Ctrl+J. You should see a copied layer in your Layers window. If it's not open, go to Window -> Layers. The shortcut for this is
F7.
Make sure you are on your copied layer. Next, go to Filter-> Blur-> Gaussian Blur.
For this example, I'm going to use 3.0px radius. Increasing the number increases its
glow effect in the end.
Finally, you go to your Layers window, and drag the copied layer (that has the gaussian blur effect) UNDER your original layer.
Lastly, go to your original layer. Inside the Layers window, locate the drop down tool on the top left. Select Soft Light.
This is a comparison of the original and the edited image @ 3.0px radius gaussian blur.
There are alternatives to .jpg. Some image uploaders and forums don't support .bmp files. To prevent quality loss, your next best options are .png or .gif files.