The parameter may need to be quoted, so you'd need to send in "beginslide=/path/to/start/slide.jpg" with the quotes around the whole thing, which may be non-intuitive?  We'd have to support named parameters in the splitting routine if we want it done using the more intuitive beginslide="/path/to/start/slide.jpg".  You could probably just do it unnamed here instead (i.e. interpret the last param not covered by a flag as the begin slide).

I think also that Left(13) will grab 2 more characters than you want :)