mod_security is an apache module that the provider can install on apache server so that there are less risks with posts. it is a sort of firewall. http://www.modsecurity.org/
Original size is done in
Resize Original Image
value is maximum size in pixels
this will only affect new uploaded pictures.
to change old pictures:
vbgallery => maintenance
Rebuild Image Sizes
and select originals.
btw, check if you dont have a space at the end of the url or the path