The size upload limit is 5M for ONTRApages. For videos in ONTRAPORT, the limit is 200M.
However, you should probably never upload an image larger than about 300 - 400 KB. And, your goal should be about 70 KB.
Here's why - larger images are compressed by the browser to fit on the screen, slowing down the response time and hurting your SEO (according to Google!) But what about all those high-resolution Apple displays? Take a cue from Apple's own site. Here's an image from http://www.apple.com/ipad-pro/ with an image that is less than 140 KB:
Here's a Technical Article from Google on how to optimize your images for the best user experience (and by extension the best SEO). That article also has links to open source (free) programs you can use to resize your images before upload.