I see what you're wanting to do.
I assume you have these tiles as some sort of grid. What you'll need to do is to do something like a for loop. You'll have look at the grid the tile was placed on, then look at the grids x-1, y-1 to x+1, y+1 and see if there are any other conveyors on those grids and change your sprite depending on where the other conveyors are.
But, you have other problems. What if a player puts down a conveyor like this
0X0
X0X
0X0
Where x are conveyors and 0 are empty.
Now, what if the player places the next one in the middle of that grid?
My suggestion is to 'guess' at what the player wants but also give them the ability to rotate the conveyors.
The easiest solution:
Have straight conveyors and curved conveyors. Allow the user place whichever they want and give them the ability to click on those conveyors to 'reverse' direction and click another place to rotate them.
One thing to consider, and this comes from a game TerraTech which also uses conveyors, when they click to reverse, it reverses all connected conveyors.
*edit
Still thinking about it, you're also going to need some way to determine which ends of those conveyors will be able to 'connect' to others. For example, a straight conveyor can connect to y-1 and y+1. If it gets rotated then it's x-1, x+1. Curved might be y-1, x+1.