That depends on whatever editor you're using. Assuming a default installation you're using CKEditor. You first have to set it in the admin/posting section to handle questions and answers and save the settings. Look for Default editor for questions.
Then click on the options link to the right of the combobox and set these settings from the plugin configuration:
Allow images to be uploaded
Allow other content to be uploaded, e.g. Flash, PDF
Maximum size of uploads