On our part, we can exclude a malfunction in uploading images.
Basically, there are 3 different ways to upload images to us - have you already tried out all of them?
a.) Upload an image from PC via Files => Upload files
In the menu, click on "Files" => "Upload files" and then click on the "Select files from PC" button.
Choose the images from your PC that you would like to upload and then click on the "Upload selected files" button.
b.) Upload an image from your PC via Editor => Insert image
In the menu, click on "Pages" => "Manage pages" and then select the page in which you want to insert an image. Click on the "Insert image" button above the editor and choose the image from your PC in order to upload it.
One click on the "OK" button adds your newly uploaded image into your page.
c.) Upload an image via the file manager
In the menu, click on "Files" => "Manage files" and then on the link "Open file manager".
By clicking the button "Upload" you can choose an image from your PC.
Important: In case that none of the above 3 methods works, please temporarily disable your virus scanner and firewall.