Ideally, the background processing of getting images should stop when a new request comes along because of a new call to WebView.getEngine().load(String)

Because it doesn't stop, I have a reproducible test case for making a vm run out of heap space:

1) Compile and run the SimpleSwingBrowser location here:
https://docs.oracle.com/javafx/2/swing/SimpleSwingBrowser.java.htm

2) Have go the url of a large image (this one is about 5.9 MB):
http://chamorrobible.org/images/photos/gpw-200905-UnitedStatesAirForce-090414-F-6911G-003-Pacific-Ocean-B-2-Spirit-stealth-bomber-F-22A-Raptor-stealth-fighters-Guam-April-2009-huge.jpg

3) while the image is loading, press the "go" button to start loading the page a few times until you run out of heap (6 times is enough javafx to fill up 256MB max heap:

Exception in thread "JavaFX Application Thread" java.lang.OutOfMemoryError: Java heap space

4) Utilizing a profiler like YourKit, the culprit at the time of the OutOfMemoryError seems to be that there are 6 "Strongly Reachable" instances of com.sun.javafx.webkit.prism.WCImageDecoderImpl that are collectively holding onto a retained size of 175MB of heap.

The retained size is originating from the the java.nio.HeapByteBuffer:

com.sun.javafx.webkit.prism.WCImageDecoderImpl is holding onto
  com.sun.javafx.iio.ImageFrame[] which continues to hold onto
  147MB of java.nio.HeapByteBuffer