you are doing several things "wrong"
firstly, you didn't clear the buffers before drawing to them. the front and back buffers are free ram areas that other things can and do use, so you need to clear them yourself with ClearDraw and ClearDraw^^r before drawing anything there or you will end up with the "junk" you mentioned.
secondly, all you are doing with that pt-on command is drawing to the front buffer. axe doesn't have a "draw to both front and back" command yet, although people have poked runer about it, so it may be added soon-ish. as of right now, you have to draw them separately, so that would mean drawing the first 8 bytes to the front buffer with pt-on(15,20,Pic1) and then the second 8 bytes in a sprite to the back buffer with pt-on(15,20,Pic1+8)^^r

hehe, it wasn't anything big
any time you have questions don't hesitate to ask! axe can be a little tricky to work with at first.

yeah, *8 is the best way to go about it.
if you wanted to loop through a list of 4 sprites, updating the buffer after displaying each one, then, you could do something like this:
for(L,0,3
pt-off(x,y,L*8+Pic1
DispGraph
End

not necessarily, as the front buffer and back buffer sprites could be stored in different places, or just stored with all the front buffer sprites together followed by all the back buffer sprites.

Generally you should put the loop before the display command, although it really doesn't matter if it's in a while loop. Make sure you keep track of your order logic, however.