If you do $content="something"; you are replacing the $content value. If you want to append something to the content, you'd need to do something like:

$content = 'initial content';
$content.= 'more content (notice the dot)';

In this case, if you want to append the image to some existing content you'd need to do

$content=the_content(); //or wherever the existing content is
$content.=$link[$i].'<br>'.'<img src="'.$src.'" alt="'.$img_title.'" title="'.$img_title.'" class="alignnone size-full wp-image-'.$img_id.'" width="500"/>';

Or the reverse if you want to put the image first, before any content.

If you do $content = "something"; you are replacing the $content variable's value. If you want to append something to the current content, you'd need to do something like:

$content = 'initial content';
$content .= 'more content (notice the dot)';

In this case, if you want to append the image to some existing content you'd need to do:

$content = get_the_content(); //or wherever the existing content is
$content .= $link[$i].'<br><img src="'.$src.'" alt="'.$img_title.'" title="'.$img_title.'" class="alignnone size-full wp-image-'.$img_id.'" width="500"/>';

Or the reverse if you want to put the image first, before any content.