This is certainly possible to do. The question is are you using the custom_index plugin? If you are it is much better.

If you can code html/php (just a little bit) you can do this.

go to elgg/mod/custom_index/views/default/canvas/layouts/new_index.php

Somewhere in there you need to add the html to call your image. I imagine you have it stored somewhere in your elgg folders. (Best to stick it in the plugin folder you are dealing with which is custom_index).

So upload the image to elgg/mod/custom_index/graphics folder

Now in your new_index.php file add the following wherever you want it to appear:

<?php

echo "<img src='{$vars['url']}mod/custom_index/graphics/myimage.whatever' />";

?>

it will appear. You can also style it inline with <img style='' src='' /> etc. Be warned, after the echo there are double quotes, so anything inside those double quotes needs to be single quotes (or vice versa).

Enjoy!