On my website, PantrySpa.com, I would like for the images

On my website, PantrySpa.com, I would like for the images to be centered at small screen widths / mobile, rather than to have text wrapped around it like on desktop. Here is an example of a post with an image in it http://www.pantryspa.com/home-and-garden-remedies/living-room-remedies/designer-decorator-secrets-decorative-lamp-shades-custom-pillows/2/ ... I still want the caption text to be to the right of the image, but I want the line with the image and caption to the right of it to be centered and to not have the post text wrap around it.

How can I center the image and caption when the screen width is 899 pixels or below (for example)?

Also, when the screen width is very small, the image is currently getting cut off... how do I fix this? I have attached a screenshot to show what I mean.

I have granted support access via the wpmu plugin, in case that helps.

Thanks for your help!