Great question. There’s not a pre-set size for the images in the blog section. Both images and videos there will scale down to fit. You’ll probably want images that are at least 760 pixels wide so they fill the full-width of the content column.
That said, for appearances, you’ll get the best result with images that are in the landscape orientation instead of portrait ones. That’s mainly due to how tall the portrait ones can be.
In the demo, most of the images are around 760 pixels wide by 500 pixels tall. For most screens that appears to be a good fit.
There is an image editor that’s built into WordPress, but if you have access to a desktop photo editor, I would recommend using that instead. If not, then go with the one that’s in the media library.
If I can help out, just let me know,
Bill