I think using proper alt tags can help. There's nothing else I can think of that can be done. You can try using structured data. I'm interested to see results.I have it showing randomly and I've verified that the image is listed with the Open Graph image pointing to the image. I do not have structured data setup yet and I cropped many of the images, so a vast majority are not the common 16:9, 4:3, or 1:1. I tend to think Google will just truncate the image if it's not a common dimension so that I'll leave for last.
I'm determined to at least tilt the odds in my favor of an image showing. Right now I'm testing structured data and using the Article type, and if not then I'll try the Review type in a couple of hours.
Well, I did something right. I'm still doing more testing but I got one image showing now with just adding structured data.
I know Google doesn't always show images, but it may if it's a product or something similar so if it shows for say 7 or 8 on the first page, there's a good chance if you're on page 1 you should be able to get it to as well.
I did go through a couple of iterations so either it took a couple of hours to take effect, or my final schema which includes the image field made the difference. I'm trying it now on a couple of other pages to further test it.