Featured image can't be uploaded sometimes at Blog options when adding an article with Helix framework. Why? What could be the problem? How to solve it?

I suppose it's beacuse of big image or something like this. However, I don't know exactly the parameter. On the other hand the publishers who will use the site to publish articles aren't well qualified so it can cause a problem in a daily basis. Could you help me to solve this?

Anyway is there any option to modify the settings of the image source of the blog. Now it uses the image which was uploaded at the Blog options however there is another tab next to it -> "Images and references" where the publisher also can upload images. Can we use this as a source so that there shouldn't be uploaded image for both places. (Some other different article listing module (called Digi Showcase) use this source to show as featured image of the article)