Image Border Issues (Slider)?

Hey!

The borders around images in a slider on my site are unpredictable - I want them to sit tightly around the images, like this:

And it usually they do, and it looks like the image above.

But sometimes I reload my page and it looks like the below, with a bit of a gap around one side of the images.

It’s probably glaringly obvious, but I can’t see where in the styling the problem is, and why the borders aren’t responsive. The 1px black border is applied to the slide-img-container div, and the slider itself is in the ‘creative services’ block, with the blue/green gradient background.

I’d be so grateful for any help, thank you!

Here’s my site, Read Only.