Add the attribute to each line of code that creates an img tag. In your text editor, search for lines that contain the string "<img".
Lines 241 and 273:
$row->item_trend_rating = sprintf('<img class="trend" src="https://wordpress.stackexchange.com/questions/96226/%s" style="https://wordpress.stackexchange.com/questions/96226/%s" width="https://wordpress.stackexchange.com/questions/96226/%s" height="https://wordpress.stackexchange.com/questions/96226/%s"></img>', $gdsr->e, $image_bg, $set_rating->size, $set_rating->size);
Becomes:
$row->item_trend_rating = sprintf('<img class="trend" src="https://wordpress.stackexchange.com/questions/96226/%s" style="https://wordpress.stackexchange.com/questions/96226/%s" width="https://wordpress.stackexchange.com/questions/96226/%s" height="https://wordpress.stackexchange.com/questions/96226/%s" alt="https://wordpress.stackexchange.com/questions/96226/%s"></img>', $gdsr->e, $image_bg, $set_rating->size, $set_rating->size, 'ALT TEXT HERE' );
Change ALT TEXT HERE to the text you want to use here.
Line 503:
$rater_stars="<img src="".STARRATING_URL.sprintf("gfx.php?type=thumbs&value=%s", $score).'" />';
Becomes:
$rater_stars="<img src="".STARRATING_URL.sprintf("gfx.php?type=thumbs&value=%s", $score) . '" alt="' . $score . '" />';
Lines 506 and 511:
$rater_stars="<img src="".STARRATING_URL.sprintf("gfx.php?value=%s", $rating).'" />';
Becomes:
$rater_stars="<img src="".STARRATING_URL.sprintf("gfx.php?value=%s", $rating) . '" alt="' . $rating . '" />';
I used $rating as the alt text because it looks like a string. Feel free to change it.