Login to Blogger Dashboard
Then choose Design --> Edit HTML
Don't forget to backup your template first.
Chose Proceed to start editing the HTML code.
Check the "Expand Widget Templates" box.
Then, find (CTRL+F) this code in the template.
<data:newerPageTitle/>
And replace it with this code,
<img src="URL-OF-PREVIOUS-BUTTON"/>
Note:Upload an image to any image hosting website and replace URL-OF-PREVIOUS-BUTTON with the url of previous button image.Now search for this,
<data:olderPageTitle/>
And replace it with this,
<img src="URL-OF-NEXT-BUTTON"/>
Note:Upload an image to any image hosting website and replace URL-OF-NEXT-BUTTONwith the url of next button image.Then search for this,
<data:homeMsg/>
above code will appear twice and replace both appearances with this code,
<img src="URL-OF-HOME-BUTTON"/>
Note:Upload an image to any image hosting website and replace URL-OF-HOME-BUTTONwith the url of next button image.Finally save your template and you are done!