Suggested size and placement
We recommend you to put images in the sub cards. Any size will work, but about 1024x1024 pixels is the most fitting size.
You can upload any image format such as PNG,JPG, PDF, GIF (yes even animated GIFs, but be aware of the data size and they won't be visible in an export) but the best performing format is PNG:s. Make sure they are Non Interlaced PNG:s if you intend to export the map.
For problems exporting a map with images in it, check out this article