Hi Andrè, thanks for your advice.
The resizing part is ok, now i can have uniform AND custom size for thumbnails customized with this plugin.
However, there's still a layout problem: for any size that is reasonable for a video-preview thumbnail (400-500px), although browsing through gallery pages is fine because actual thumbnails get downscaled with height and width arguments of <img> tag, the filmstrip in the displayimage.php page you can browse next and previous videos with is unusable, because it displays full-size thumbnails (as specified replacing $CONFIG['thumb_width']), and this leads to no more than one thumbnail displayed correctly instead of 5, which are displayed cut.
Any more advice?
Thanks anyway =)