Once you have selected an image that reinforces your message, it should be sized to fit the intended location and should not exceed 100KB. Keeping file sizes small will ensure that your webpage or online document will load quickly. For guidance on sizing images, click here.
Re-think using animated gifs. Since these moving images cannot be "turned off," they are more likely to distract and irritate.
Please be aware that automatic display of mixed content, such as slide shows, is currently blocked by a number of browsers. You may be better off including still images than risk having your visitors see a perpetual "loading" message.
From the Google Docs training module (and modified for Google Apps EDU):
To insert an image, follow these steps:
Once an image is added, it can be resized (by choosing from S/M/L/original) and aligned (left/center/right) by clicking on it and using the small horizontal pop-up menu.