How do I position an image at the bottom of div?

Add relative positioning to the wrapping div tag, then absolutely position the image within it like this:

CSS:

.div-wrapper {
    position: relative;
    height: 300px;
    width: 300px;
}

.div-wrapper img {
    position: absolute;
    left: 0;
    bottom: 0;
}

HTML:

<div class="div-wrapper">
    <img src="blah.png"/>
</div>

Now the image sits at the bottom of the div.

Using flexbox:

HTML:

<div class="wrapper">
    <img src="pikachu.gif"/>
</div>

CSS:

.wrapper {
    height: 300px;
    width: 300px;
    display: flex;
    align-items: flex-end;
}

As requested in some comments on another answer, the image can also be horizontally centred with justify-content: center;