Resized thumbnail images not visible on live site

The thumbnail images displayed in the home page’s news item column are all the same width, but some are very tall, making the whole thing look messy and unprofessional. I have therefore been trying to edit each thumbnail image, as well as changing the settings for how future uploads should look. But the edited images are not showing on the home page. I have cleared the browser cache and our hosting company has restarted the site service, but the changes are still not visible – the old image sizes are still there.

This is how I attempted to edit the images:
1. In the media library, I have selected the image in question and clicked edit image. (I have also tried editing the image from the page where it is used, but since it brings up the same edit window I don’t suppose it makes any difference where the image is accessed from.)
2. I have cropped the image and made sure to select the radio button to apply changes to the thumbnail only before saving the image.
3. I have then opened the page that has this image as its featured image, removed it and then re-added it in an attempt to clear any temporary files that may have saved the older version (which I saw in other posts from users with the same issue may be a problem).
4. I have clicked update to save the page.

I am starting to wonder at this point whether the thumbnail images are used at all on the home page, or whether it’s the original upload just resized to fit a specific width.

Or whether the theme saves the thumbnail image somewhere else so that manually edited thumbnails just don’t work because they are not overriding that initial saved image. In which case, does this mean I need to install a plugin to force refresh thumbnails? I was really hoping to avoid this as we reckon most thumbnails need to be manually edited in order to get the best out of them – often the key part of the image that we wish to show in the thumbnail is *not* in the centre, so an automatic crop would not work well.

The source code for the page doesn’t give any indication that the image is saved anywhere else than where initially uploaded to (wp-upload/date etc.), but *does* suggest it’s taking the original size image and re-sizing it to a specified width, rather than using the dedicated thumbnail size – why would this be?

Is this a theme “thing”? (I thought so, hence posting here – apologies if I’m wrong.) Or something I should speak to the developers who created the site about?

Many thanks.