Google Discover generally should not show content or images that Google's SafeSearch filter would block. Danny Sullivan, Google's Search Liaison, said this on Twitter a few weeks ago but this was implied by previous statements from other Googlers over the years.
Danny Sullivan was asked by Lily Ray, "Is the below article on SafeSearch filtering the best place to look for guidance on Google Discover? Seems that sites with *some* adult content may be excluded from Discover entirely; does this guidance apply?"
"It's pretty likely SafeSearch applies to Discover, so yes. Will update later if that's not the case," Danny Sullivan replied. And he said that a couple of weeks ago, so since there was no update, we can imply this is the case.
He later said, "our systems, including on Discover, generally don't show content that might be borderline explicit or shocking etc. in situations where people wouldn't expect it."
Here are those tweets:
It's pretty likely SafeSearch applies to Discover, so yes. Will update later if that's not the case.
— Google SearchLiaison (@searchliaison) April 18, 2023
If anyone thinks content shouldn't be filtered by SafeSearch, yes, that form can be used. But our systems, including on Discover, generally don't show content that might be borderline explicit or shocking etc. in situations where people wouldn't expect it. That's kind of covered…
— Google SearchLiaison (@searchliaison) April 20, 2023
Previously, Gary Illyes and John Mueller from Google News, Top Stories, & Discover do have policies and mentions of using such words and that it can impact the visibility of your content in those areas. We covered those policies in 2021 when they first came out.
If you ever see explicit images in Google Discover, it is likely not being picked up by Google's SafeSearch filter.
Forum discussion at Twitter.