By default, an image is rendered inline, like a letter so it sits on the same line that a, b, c and d sit on.

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There is space below that line for the descenders you find on letters like g, j, p and q.

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Demonstration of descenders

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You can:

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adjust the vertical-align of the image to position it elsewhere (e.g. the middle) or

change the display so it isn't inline.