This tut will teach you how to change skins in minecraft. (downloaded version not online).
First you need to find where the Minecraft files are located.
In Windows. Press Windows + R (to open the run function) then type in. "%appdata%" (without the " Quotation Marks).
Then there will be a folder named .Minecraft.
The bin folder. There will be a minecraft.jar.
Open it with Winzip (or 7zip) (rename to .zip) and replace the char.png file with the file image you edited.
Rename the .zip file into .jar again after you replaced it, then now, you can play minecraft (the downloaded versoin) with your new skin :D