I am noit infront of my computer, but in brussels at eu summit.. reading mails in internet-cafe.
I cant tell for sure, there is such a limitation in vbulletin also. i think in config.php.
but in vbgallery the debug part just says the image is to big.
It then calculates the size of image that would fit with your settings.
So this meens you could upload bigger images in pixels, but the should be smaller in filesize, as your server cannot process them.
the 3 or 4000 pixels just apply to this image.
It also depends on dpi settings of the pic, etc.. etc..
I would suggest, as chuck said, to use imagemagic, but depending on your server settings I do not know if it will allow processing of much bigger images. (depends on memory instralled to server..