I just tried it, and… Yes! It is working with “large”.
And then I tried with another custom size “DFX-Mini”, and it worked also.
But it is still not working for size “DFX-Header”… I noticed, using an ftp program, that the size is of DFX-Header is not even generated for the images, so that’s why it didn’t work – the image doesn’t exist. Not sure why.
I have it working with “large” and that is good enough for me, so I’m closing the issue.
Thank you for the tip!