Toggle based on checkbox
Personally, I don't like the first two lines of code, but I'm aware that you're only concerned with this function and I wouldn't know how to improve those two lines without seeing the calling function. However:

Do the checkbox and rows share a common (and close) parent?

Example markup:

&lt;tr&gt;
    &lt;td&gt;
        &lt;img src="../layouts/15/images/finished.png" /&gt;
    &lt;/td&gt;
    &lt;td&gt;
        &lt;input type="checkbox" value="some value"&gt;
    &lt;/td&gt;
&lt;/tr&gt;


In this case, the img and input share the same tr. Meaning you could have this instead:

var $row = $('tr img[src="../_layouts/15/images/' + img + '.png"]').closest('tr');
var show = $('#' + checkBox, $row).prop('checked'); 


Specifically looking at this code: $('#' + checkBox, $row). Here we are looking for an element with the id specified within the context of $row. The advantage of this is that jQuery doesn't traverse the whole DOM looking for that input, it only looks within the tr. This won't make much of a difference in this case - but it's important to have this approach with performance in mind on client side. You may have also noticed that I renamed rows to $row? A lot of people will prefix a variable with a $ if the variable is a jQuery selector, this gives other developers a bit more visibility.

Ternary operator

Instead of your if/else here you could use a ternary operator to reduce the lines of code:

show ? rows.show() : rows.hide();


Naming variables

The function variable names are a bit ambiguous in my opinion. To me checkBox suggests I'm actually receiving a checkbox not the ID property. The same with img applies here. More suitable ones might be:

function toggleRows(checkboxId, imgName)