It appears that Google has increased the number of times they are showing image thumbnails in the mobile search results snippets. A report by RankRanger, which I covered yesterday at Search Engine Land, shows that 75% of the search results they track have at least one image thumbnail in the results – which is up from 40%.
While Eric Hedekar from Moz agrees with the 75% figure, he said he thinks it has been that way for some time. Although the number of image thumbnails have gone up a lot on a specific search result set.

No substantial bump in # of SERPs with image thumbnail results on our end. Still holding steady around 70-75%. I can confirm that we saw a near doubling of the amount per SERP on the 7th, but it’s been decreasing since then (no bump today).
— Eric Hedekar (@EricHedekar) March 11, 2019
Google has been showing image thumbnails in the long time now. Google even is testing showing them on desktop results.
In any event here are RankRanger charts, this first one shows the increase in how often they show up for any given query:

The second one shows the average number of mobile Image Thumbnails on page one jumped to five on March 11th:

Google gave us a statement at Search Engine Land “We’re always working on new ways to enhance Search with helpful visuals, and we encourage site owners to enrich their pages appropriately.”
Here are some screen shots of what this looks like:

Forum discussion at Twitter.

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