In the top right corner click on “Create“ → “Image Creative: Hosted”.
A dialog will open, here you can just select multiple files to upload. You will realise that the name field will change to uneditable, because all creatives will use the file name as default value which can be changed later.
When clicking on “Save” all images will be uploaded and image creatives are created. This might take some time due to the size of your images, please be patient and don’t leave the page.