Cloud assets such as images or CSS stylesheets can be stored in the portal and are found in the IDE page. This is can be accessed by Administrators in two ways - via the dropdown at the top of the page as shown in the image below, or via URL (https://<yourportalurlhere.com>/sys/ide/index).
Once in the IDE page, click on the 'Assets' icon (third on the left side as shown). Here you can edit or delete existing files by right-clicking them, or upload new ones by right-clicking a folder and selecting 'Upload Asset(s)'.
An important note - if you upload a file with the same name as an existing one it will overwrite the old file. If you have two files with the same name and wish to keep both, rename the new file with a different filename before upload.
For more information on how to insert cloud asset images to article posts and email templates, see this documentation.