Beginner Questions and Feedback

Hi all,

I'm currently writing my second script. 

The first one was simply pressing a key. It works on the first run. Picture was taken by the mobile build-in screen capture.

1. Question:
You use PNG for the images.  Does it mean its fully supported and I can use PNGs with transparency channel? (Find the banana)

2. Question:
Now im trying my second script: 

_{while (true) do
    wait(3)
    testBraun = findAll("braun.png")
    for i, m in ipairs(testBraun) do
        click(m)
    end
    wait(1)
    testBlau = findAll("blau.png")
    for i, m in ipairs(testBlau) do
        click(m)
    end
    wait(1)
    testGelb = findAll("gelb.png")
    for i, m in ipairs(testGelb) do
        click(m)
    end
    wait(1)
    testRot = findAll("rot.png")
    for i, m in ipairs(testRot) do
        click(m)
    end
end}

And it works! :-) ...until the third loop :-(

_{Runtime error:
com.appautomatic.anlulua.f:
Can't find P(gelb.png) S: 0.7
stack traceback:
 [C]: in function'findAll'
 ?: in function 'findAll'
 /sdcard/AnkuLua/xxx/xxx.lua:25:
in main chunk}

Does it mean the source image was not found? (It exist) Or was no matching found? What can i do in that case? Some kind of exist or null check?

BTW: It was inspired by ankulua.boards.net/thread/6/objects-methods-introduction-sikuli-compatible -- The first row in the first script after findAll contains an error ;-)

Cheers
Curval