How about placing a loader while the image is uploading? I've tried to upload a large image on the demo page, no indication whether the image is uploading or not.

Hello Firdaus,
Please define larger image? How big/what dimensions? Probably you are just hitting the limits set on our server.

JPG size 4mb, 7360 x 4912.

I see it loads on MediumEditorAdminDashboard demo page but not on MediumEditorCompanyBlog demo page.

Yes, because the company blog demo uploads and then resizes the image, while the other one embeds it.
Will check the limits set for our showcases, but probably you are just hitting them.

There should always be a possibility to show a progress bar combined with upload?

Currently there is no progress bar available in image upload add-on. I will check with our developers if this can be integrated in the next updates.