How to change xvfb-run permission?

Question

X Window System

Hey!
Unable to change resolution when using xvfb-run.
I do this:
xvfb-run -s '-screen 0, 1366x768x24' ./capt --url= someurl.ru --out=someimg.jpg
where ./capt is a self-written utility https://github.com/allush/capt (I wrote instead of cutycapt, which has not been supported for about a year and is not built under ubuntu)
Regardless of the -screen parameters, the image is 624 pixels wide (xvfb-run uses 640x480x8 by default).
Different variations of permissions and writing command parameters did not help (I mean write --server-args instead of -s)
What could be the matter?