I feel your code can be improved but using what you have try this:
/* * Set common variables */
$width = ( isset($this->conf['width']) && (!empty($this->conf['width'])) && intval($this->conf['width']) > 0 ) ? $this->conf['width'] : 320;
$height = ( isset($this->conf['height']) && (!empty($this->conf['height'])) && intval($this->conf['height']) > 0 ) ? $this->conf['height'] : 240;
if (isset($this->aCurArgs['width']) && !empty($this->aCurArgs['width'])&& intval($this->aCurArgs['width']) > 0)
$width = trim($this->aCurArgs['width']);
if (isset($this->aCurArgs['height']) && !empty($this->aCurArgs['height']) && intval($this->aCurArgs['height']) > 0)
$height = trim($this->aCurArgs['height']);
$src1 = ( isset($this->aCurArgs['src1']) && !empty($this->aCurArgs['src1']) ) ? trim($this->aCurArgs['src1']) : false;
$src2 = ( isset($this->aCurArgs['src2']) && !empty($this->aCurArgs['src2']) ) ? trim($this->aCurArgs['src2']) : false;
i added the check for 0 to the two IF
.