Paulo, please take a look at do_svy.htmpl...the way the image button is present in the survey page it the same as the rest of the interface(imagebutton.js) and change its size is not an easy thing. You could change the button image easily
by change
usdImg["circleca"] = "$CAisd/img/circleca.png";
in that file but not the size. What you could do(not test here but just for your consideration) is to display your image not the interface image button by adding something like
<img src="$CAisd/img/myimage.gif" width="42" HEIGHT="42" alt="my company logo">
at the beginning of the survey page. Thanks _Chi