Parallel Flash Loader issues

As you can see from the PFL source, the upper 16 header bits contain also an image length information. If it's not set accordingly, the configuration load must be expected to fail. This point is also involved with "know, what to write to the first bytes". I didn't yet try to write a PFL flash image by user code, so I can't tell you precisely where to find the respective information. But I'm sure you can get it, also other embedded configuration methods are able to retrieve an image size information.