Target image size with @media?

[John]

I want to target image sizes over 250 to be centered when the screen width is below 767px. I could add the attachment ID at the end, but if new images are added, then I’d have to add it in. I’m using WordPress and bootstrap 320 theme.
I have this so far, which targets all attachments:

@media (max-width: 767px) {
    div[id*='attachment_'] {
        margin: 0 auto; 
}
}

@media (min-width: 768px) {
    div[id*='attachment_'] {
        float: right; 
} 
}

[John]

Here’s the site.
It’s a work in progress.

Original site is here.

[Author]

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