Reduce padding between blog post images

I am wanting to reduce the padding between the images on my blog posts. I asked my theme creator how to do that in the file editor, and he said it was already as close as possible. But I’m not sure how that could be the case? Isn’t everything done through css? What could I add or look into to fix this? I am familiar with css and don’t mind doing some extra coding to make this work. I’d like 5px of white space between the photos, instead of the 16px space it currently seems to be. Thank you!

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