Some images don't show

I’ve been helping a friend get her site set up over the past week and have encountered an annoying problem, which until yesterday seemed to only be happening to me but now it’s occurring when she looks at the site as well. Several of her images don’t show up on the site where they’re supposed to: one of the images in the slider on her home page; one image in the middle of one of her posts created with the help of SiteOrigin’s Page Builder plugin; and one in a gallery. I haven’t looked through all the pages to see if there are others, so there could be more. Last week I experienced the problem to a much greater degree that included some images appearing as avatar-sized thumbnails in addition to several that didn’t show up at all.

My friend has looked at her site in both Firefox and Chrome, and found the problem existed in both, suggesting it isn’t a browser issue. Does anyone have ideas about what’s going on and what I can change to fix it? This site is heavily dependent on the images helping to tell the stories so it’s kind of important we get it worked out quickly.

The URLs to the pages where I’ve seen the problem are:
https://simplemindedsims.com (the slider is at the bottom of the page and the problem image is the last slide)
https://simplemindedsims.com/2015/06/barney-beake-sci-fi-madness/ (the problem image is the 5th or 6th one down)
https://simplemindedsims.com/2015/07/silly-sims/ (the 1st image in the gallery doesn’t appear on the page, but will show up in the lightbox if it’s clicked on)

TIA for any and all help with this.