Default Classic template does not allow to display menu thumbnail image on front-office. To display menu thumbnails in the main menu you need to just follow the below steps.

Go to: Theme root directory > modules > ps_mainmenu > ps_mainmenu.tpl

Now, Copy the below code.

                            {if $node.image_urls|count}
                                
                            {/if}

Then place it to just after below line.

                            {menu nodes=$node.children depth=$node.depth parent=$node}

Modified file will be look like as below

{assign var=_counter value=0}
{function name="menu" nodes=[] depth=0 parent=null}
    {if $nodes|count}
        
    {/if}
{/function}


Now save the changes and clear the cache after uploading menu thumbnails image from the back-office. Your uploaded menu thumbnail image will be visible in main menu on front-side.

menu-thumbnail-bo

Front-office view:
menu-thumbnail-fo

That’s all.