Is it common practice to insert images at the top of a page?
You are asking a question about page layout. What is common may not be what is best for readers to understand. One of the issues with having a picture in the middle of text is that you often end up with a few lines of text above or below the figure. As a consequence, the eye often has difficulty distinguishing between what's part of the figure and what's part of the regular flow of text. That's particularly awkward if it's just one, two, or three lines of text. This does not happen if you put figures at the top or the bottom of the page, and consequently that's what most style guides recommend.