can't get rid of left margin inside text box that contains image

[enginist]

I'm enclosing my images inside of text boxes so that the images and their descriptive, non-numerical captions (such as "the telescope") will be tied to the image. The problem is that I can't fit the text box border snugly around the image (+ caption) because there's a left margin inside the text box whose width seemingly cannot be reduced. I've tried clicking on the box >> Format Shape >> Text Box >> Internal Margin: Left: 0", but the margin will not go away. How can I get rid of it?