The distance between a text wrapped image and the text above and below is entered in the Text Wrap Panel
BUT if your text is aligned to the Baseline Grid, this will override the bottom distance, and the next line below your image will always jump down to the next baseline. If you have this, you must size the picture to have its height as a multiple of your baseline grid distance.
This is caused by leading. At the top of your image the wrap is intereacting with the bottom of the leading slug which is cloer to the baseline for the type than the top of the slug is. If your wrp also goes into a line of text, that line is skipped and you see the next line below. Unchecking "Skip By Leading" in the composition section of the preferences will reduce the discrepancy (by always postioning the next line of type below the wrap with the top of the first leading slug even with the wrap boundary) but it will throw the line possitioning off if you are depending on leading to maintain level lines across columns or to line up at the botom of adjacent pages, much the same as adding paragraph spacing.
You can maipulate either the amount of wrap in the tow sides, or adjust the position of the image manually to make things more even.