It works fine here and looks great.
That screen shoot is probably something to do with that particular PC. The thing with blitz is that it can have certain errors when you do certain things like draw stuff off screen. Its some sort of memory overflow or other. Somtimes you get it from updated gfx drivers. Chances are if you used an older driver, or even the latest driver if ur using an old version already, it would fix the problem.
It is possible to resolve some of these issues by the way you progam the application too.
I noticed this line of code
Else If l\x > GraphicsWidth()
possibly chaning that to
Else If l\x >= GraphicsWidth()
and then moving where you actually draw the image to somewhere after the range checking mite clear it up.
What thats saying to me is that at somepoint, DrawImage is drawing an image at position GraphicsWidth() wich technically memory wise doesnt exist.
Blitz should actually cope with this, and it does most of the time. However on the rare occasion you do get some wierd bugs like that.
I got no idea if my suggestion will have any impact on the result, but its worth a try. I cant unfortunately replicate the problem. But I stand by that advice in that you try to keep the poition you draw anything within the range of 0->GraphicsWidth()-1 and 0->GraphicsHeight()-1 and do the checking before you actually draw anything.