The images should be focused on the content and not include unnecessary parts of the UI ex. header / empty canvas etc.
Gifs can be recorded using the a screen recorder like loom and converted to a gif using gifski. Gifs should be of 26 - 30 fps and be of high quality. If you do not have access to a good gif converter, please upload the video as is and raise a PR. We will be happy to help with Gif creation!