Go to the Assessments section on the left and click on the assessment that you want to add an image to.
Go to the Questions tab. Click on +Add Questions.
Click Advanced Options. Choose the file that you want to upload by clicking Choose File. Click Save Question.
Here is what an image will look like.
If you want to add animated GIFs, you will first need to add it to your Materials section, then use the Attach Existing Material option to embed it.