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Creating a custom image such as this can be done with programs like Adobe Photoshop among others. If you have an artist on staff, you can be as creative as you want. Common image types like .jpg or .png files are supported to allow images with (.jpgpng) and without (.pngjpg) transparency. Images with transparency will allow for more creative shapes for infopopups such as the leaf example above. Once you have an image ready to go, it will need to be hosted online someplace. Once hosted, simply add that custom html address to the configure window and set the height and width dimensions to match your picture's. This can easily be discovered by right clicking on the image in a web browser and selecting "Inspect Element" if you don't know what it is.
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