Personally, I've never come across anyone nofollowing such links. That doesn't mean it's wrong to nofollow them; that all depends on your particular case, as Stephen alluded to in his question.
When you nofollow a link, you're usually doing it for a couple of different reasons:
- You don't want to signal to search engines that you trust this link, or you simply don't want to be associated with it for some reason.
- You don't want to pass the link juice you mentioned, preferring to keep it for your own page instead of spreading it around.
In Stephen's example, user-generated content is a good use case for nofollowing. Actually, you'd want to nofollow any link your user posts, such as links in replies on a message board, because you have little to no control over what gets posted.
Now, if you have a blog post, and you've linked to a meme here, an image from another website there, I'd say don't waste your time nofollowing. If your page only links out to a few images, you're unlikely to see a difference in rankings, because the loss of link juice will be miniscule. You have to link out to a lot of stuff to really lose link juice from your own page.
One of the companies I worked for had a website with a Leadership page. The photos of the dozen or so executives linked out to their LinkedIn profiles. LinkedIn is a reputable website, though a lot of their content is behind a login wall. I nofollowed all those links, just to see if saving the page's link juice would drive it up in SERP's. It did nothing, probably because there were only a dozen profiles and the page's content was on the weak side anyway. I realize this is a different linking structure from yours; I'm just illustrating the point that in cases of a few images here and there, you're unlikely to move the needle either way.
So, for your particular linking structure:
- Don't worry about nofollowing on a regular website page, unless you want to distance yourself from that link.
- Nofollow on user-generated content, or on a spammy-looking page that's all links. (Or, avoid creating that page in the first place.)
- Be mindful of the source where these images come from.