Images

When you load up an image from you computer, the image is read locally in your browser and converted to Base64. This is a long string that is stored in the browser and also by extension in the importable JSON so that it can travel with the rest of the script. When you export, the Base64 string is converted to binary so that it can be used inside ScriptUI.

You should resize images before loading them in the app. There is no built in resize in SDB and while ScriptUI can do it, I chose not to include that here in favor of a smaller file size.

I wouldn't recommend using large images. Standard Icon sizes are probably fine. There aren't any limitations really, but images are pretty costly when it comes to file size. Not just that... Large images can make the export very slow and possibly fail.