In this tutorial I will explain how to apply anti-aliasing to your render, this isn't required for all renders but sometimes it will be very necessary. This tutorial applies for most GFX software such as Gimp, Paint.NET or Photoshop.
First of all, I'm going to explain what anti-aliasing is, if you take a look at this render:
You will see that the edges of the render are a bit sharp and pixellated, this does not look very good and it doesn't help blending in. This was mainly caused by resizing it.
Now this is what it looks like after applying anti-aliasing:
First of all, grab the render that doesn't blend in nicely with the background ( note that this technique doesn't work with renders that have white edges, this means that it's just badly rendered ), and open it in your software of choice, I'm going with Paint.NET. Put it over a white background.
Now select the whole render by pressing ctrl+A and copy it by pressing ctrl+C, then create a new layer and paste it, now you should have two copies of the render.
Now select the render on the lower layer and apply a gaussian blur.
Make sure to select the radius as 1
You should now have something that looks like this:
Now we're almost done, to avoid that it looks like it's glowing, we want to decrease the opacity to 50%.
On Paint.NET I put the opacity to 150.
Now the edges should look smooth, you can delete the white background layer, flatten it and use it for whatever you need it for.
I hope this tutorial has been helpful for those people who have always wanted to use that one render that just didn't blend well, I might be doing more tutorials.