You have a permission problem.
If you look at the image for screen size 1200px and above and try to open it directly, it will show.
But if you try to open directly the image for 992px and above, it asks you to log-in to access it. So when logged out, you have the issue you're describing.

I'm not sure if this is the problem, because I've tested this with being logged in and out, I get the same results, I just tested this now while I was logged in and the images didn't show again. Could it be another thing causing it?

There might be other reasons for the problem but since I don't have permission to see the file I can't say.

Here's what I can tell you with total certainty:
1- Those files are not accessible to everyone, only people with the proper authorizations
2- being logged in is not enough because you could be logged in as someone who also doesn't have the proper authorization
3- the only user who is never limited in any way is the superuser (the admin)

So if you want to make sure, log in as the superuser and see what happens. If you still can't see the images then yes, you have another problem.

Hi mnakalay, I tried logging in with the admin credentials and the same issue happened, this issue didn't happen before when we had the Concrete5.7 but it happened when upgdraded to Concrete5.8, do you think this could be a problem in the core files?

Yes at this point it could be anything.
- Your update missed some files
- something in your theme's custom code is doing it
- any other possible reasons that would require investigating and running some tests

Hi mnakalay, I did more tests today, I compared the images that were showing and those that weren't showing. I found that those images that were showing were 750px in width, while those that didn't show were 640px in width.

So I increased the sizes for those images to 750 instead of 640 and re-uploaded them again, it solved the problem and they showed with no problem.

The summary I found for this problem was,
- images that are 640px width, they don't show in screen sizes between 992 and 1199
- images that are 320px width. they don't show in screen sizes between 320 and 599
- images that are 750px and higher are showing in all screen sizes.

Not only those images don't show, but they don't even exist on the application/files/thumbnails folder. I traced the file paths for those thumbnail that don't show from the developer tools in Chrome. I found that they don't exist (not generated) in the thumbnails folder. So now I'm sure it isn't a file permission issue. What do you think could be causing this issue?

ok, so what probably happened is those images were already in the file manager when the thumbnail type responsible for that particular size was created.

You need to rescan your images so all sizes are generated.

It is a bit tedious to do it from the dashboard if you have lots of images so I suggest you use this free add-on that will do it for you:
http://www.concrete5.org/marketplace/addons/rescan-all-files...

Hi mnakalay, I tried your suggestion but it actually didn't work, the images are still now showing in some particular screen sizes, when I upscale the image to 641 instead of 640 it actually works alright, I think this is some bug in Concrete5

It doesn't sound right. Concrete5 doesn't care if an image is one size or a pixel bigger.

But as long as you found a solution it's fine.