Calculates the file size of the new image resized from the image of a specified index in buffer.
.GetImageSize(sImageIndex, iWidth, iHeight);
sImageIndex: specifies the index of image in buffer. The index is 0-based.
iWidth: specifies the new width.
iHeight: specifies the new height.
<double>: returns the size in bytes.
If Dynamic Web TWAIN fails to calculate the size of the image. -1 is returned.