Minel website template, images in flexslide not responsive

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I would be really grateful for some help. I have a very limited understanding of HTML and am trying to customize this webpage template using HTML/css. My main issue is that when I add flexslider into the template following the instructions provided in the documentation support, the images I add to the template are not responsive. Here are the instructions for adding images to the slider:

<div class="bt-flexslider flexslider">
	<div class="slides">
		<div class="flex-slide">

			[...]

		</div>
		<div class="flex-slide">

			[...]

		</div>
		<div class="flex-slide">

			[...]

		</div>
	</div>
</div>

The section in which I am trying to add the images has the following code:

I apologise in advance if this a completely dumb dee dee dum dum question, but I am completely out of my depth here and any advice/help would be gratefully received.

Thank you in advance and Merry Christmas!