Before you hit the return button, make sure it is where you want it to be. Normally when you upload more than one image, the cursor is at the end of last image. To make space between two images, you should bring the cursor to the end of the first image and then press return (twice - once to bring the next image to next line and second time to create the space).
You have to do this for each image or better still what I do to avoid this is -
. Set the cursor at the beginning of the line (it will be at the end of the text if you typed any)
. Insert Image one. The cursor is at the end, so press return twice.
. Insert Image two. The cursor is at the end, so press return twice.
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. Insert last Image. The cursor is at the end, so press return once.