Notion limits image uploads on the free plan to 5MB. You can create a shortcut on iOS to convert images to JPEG and reduce the file size. This even works if the image is already in JPEG format. Unfortunately you cannot specify an ending file size. The slider for quality is the only way to change the resulting files size.

Use the shortcuts app to create the following:

https://s3-us-west-2.amazonaws.com/secure.notion-static.com/3b11c476-feca-4841-9df9-a3bf12cea1bd/76C21C7F-4910-4892-9247-B1D61E34D99C.jpeg