How to add custom content (text/image) at start of content (IN content ie the same line)

You can try the following:

! is_admin() && add_filter( 'the_content', function( $content )
{
    if( in_the_loop() )   // <-- Target the main loop
    {
        $prepend = 'HERE IS SOME CUSTOM TEXT'; // <-- Edit your text here
        $content = $prepend . $content;
    } 
    return $content;
}, 9 ); // <-- Choose some priority < 10

where we choose the priority before the content is taken through wpautop.