RESIZING CUSTOM FIELD IMAGES

Hey everyone.

I was hoping someone might be able to point me in the right direction,

I have set up a custom field for “logo image”, so the client can update this whenever they like.

Now i’m up to media queries, i cannot figure out how to resize the custom field image.

It is placed in the php as:

<div class=”section1logo”>
<img src=”<?php the_field(‘p1logoimage’); ?>” />
</div>

I have tried a heap of css to affect the image, to no avail, including:

.p1logoimage {
background-size:400px auto;
}

.section1logo {
background-size:400px auto;
}

Will trade good vibes for suggestions/advice.

Thanks!