you can try this:
if(get_field('activity_rating') == "1") {
echo 'path/to/image/onestar.jpg';
} elseif (get_field('activity_rating') == "2") {
echo 'path/to/image/twostar.jpg';
} elseif (get_field('activity_rating') == "3") {
echo 'path/to/image/threestar.jpg';
} elseif (get_field('activity_rating') == "4") {
echo 'path/to/image/fourstar.jpg';
} else (get_field('activity_rating') == "5") {
echo 'path/to/image/fivestar.jpg';
}