Why an image field shows a button properties page and not working like image in LiveCycle Designer

All of the code for loading the icons is in a document script, which executes when the PDF is opened. Performing annotation related actions (form fields are annotations) during the open process is problematic because the PDF may not be completely loaded and initialized at the time the script is run, which is why document scripts should only be used for defining functions and variables.  This is especially problematic if complex operation are required such as image conversion.  

One solution to this is to delay execution of the code by using the "app.setTimeout" function to call a document level fucntion that performs the process. 

Another solution is to use a Page open action on the first page to call the initialization function. If this method is used you also have to provide a state variable to block the function from being called multiple times.