Its very easy and simple to change the logo image in Magento. In default Magento theme logo image is picked up from the {{Magento_Root}}/skin/frontend/default/default/images/logo.gif.

One option is to create gif image of you logo and name it logo.gif and upload it in the {{Magento_Root}}/skin/frontend/default/default/images/  folder to overwrite the old logo image file.

If you have a logo image in a different format say ‘png’ or the logo image file has a different name then follow the below mentioned steps:

1. Upload the logo image in the folder {{Magento_Root}}/skin/frontend/default/default/images/
2. Login to Admin and navigate to  System >> Configuration >> Design
3. Open the “Header” section
4. Now update the logo image path. If the logo image file name is “logo_techawaken.gif” and is placed in the themes “images” folder then you need to change the “Logo Image Src” to “images/logo_techawaken.gif”. Also change the “Logo Image Alt” text.
5. Click “Save Config” to save the configuration.
6. Refresh the Magento Caches
7. Reload you webpage to verify the logo change

If you are using a custom theme then you have to upload the logo image in {{Magento_Root}}/skin/frontend/{{custom_package}}/{{custom_theme}}/images/  folder in Step 1 above and rest of the steps remain the same.