What you need
1.Decal Converter (Download Here)
Once you've downloaded Decal Converter, extract it to your desktop or somewhere you can easily access the progam. Once you've extracted the zip archive you'll see serveral files, We'll only need to use DecalConverter.exe, so go ahead and double click it.
We are going to start with the image below.
With Decal Converter open, click File on the top left corner and select open. Browse and open the image you want to convert, Below is what it looks like so far.
Now we want to make the spray transparent (remove the gray around the terrorist). Under Transparent Color you should see the box is automatically colored gray. Click replace then select Max for both Width and Height, The image should now have a blue background.
Now we we're ready to convert the image to a decal, Click Decal on the top of the window and select Make Decal. For easy accessibility, click Add then select your desktop. This makes it easier for you to find the converted decals.
After you select the OK button you should end up with a file called pldecal.wad - this is your spray decal. Browse you your cstrike directory, usually:
Since Steam's release the file is no longer called pldecal, so we must rename it to tempdecal. Go ahead and rename the file to tempdecal, after you've renamed it, drag the file to your cstrike directory. So we don't accidently overwrite the spray while in the cs options menu we make the file Read-Only - to do this right click tempdecal and select properties then select the Read-only checkbox and click OK. Should see something like this.
Time to test our spray logo. Launch CS and join a server and test. Our final spray looks like this.
Notice we're showing you the spray with and without transparency. It's up to you whether to use transparencies or not.