You can control the amount of space under each image in a list by adjusting the top margin:
.dokuwiki div.li img.media {
margin-bottom: 3em;
}
Put this in conf/userstyle.css. This will only affect images in lists. You may have to experiment to get the best result for different size images. Each time you change the em size, refresh your browser cache (CTRL+SHIFT+DELETE). Attached is an example at 3em.