Hi in my experience with X3 i found few issues or may be bugs and one of them is resizer of images.
If i post large images ( for example 9449x3543 12M ) the resizer is do nothing - the images are uploaded as is.
But if i resize them in ACDSEE to 50% - so them become 4752x1772 4.38M, than resize to 1600 and 95% quality is working.
Also i tried to post original large JPEGs from my CANON 5D Mark III - the images are uploaded without any resize.
Can someone please check the problem ?