Hold added image position in R markdown

Question

I want to add an image in the middle of R markdown report, but when I knit it to pdf, the image appears on the top of the page, not in the intended place. Here is what I use in R markdown:

![help_dataframe_screenshot](help_dataframe_screenshot.png)

Anyone knows how to fix it? I need the image at where I insert it, not on the page top.