There is no direct function, but there are some easy ways to achieve what you want (what is most suited for you, depends on your purposes):
- Align the top of all caption under the bottom of your pictures (there is a button for this in the alignment toolset). Then mark all of your captions and move them downwards by your desired gap length.
- Create a dummy object (e.g., a borderless rectangle) with the desired gap length as height. Align this object’s top at the bottom of your picture, then align the captions top at the bottom of said object. (Repeat so for every object.)
Be aware, however, that the best anchor for aligning text is not its top, but its baseline. E.g., the distance between the top of the text and the bottom of the picture should be smaller if the first line of the text contains an Ä, À, Å or similar than if it does only contain lowercase letters without ascender. The best way, to deal with this problem is to make all text identical dummy texts first and change them only after they are aligned.