If your users don't see the Upload tab when inserting images in the WYSIWYG or browsing for files in a file chooser, you may need to adjust some Site Role abilities.
Users need these two Site Role abilities enabled in order to upload files via the file chooser:
Workflow: Bypass workflow
Upload: Upload images in file chooser
You can check out a User's effective abilities for a Site by following these steps.
If you don't wish a User to be able to bypass workflow, they'll need to upload Files beforehand using an Asset Factory and browse for them in the file chooser.