A problem that has been the bane of wIMG for quite a while already was the black boxes that sometimes appeared on certain screenshots right in the middle.
So far I had not found the cause of those black boxes despite lots of searching and testing. However a few days ago I found this:
There may be a bug or at least a glitch on some systems, as import -screen and import -window root may show “black boxes” overlaying the window that has been captured, which indicates that some other window(s) are getting on top of it before the capture takes place, even if you don't see it/them!
I don't know what causes this behaviour, nor have found or really searched for an explanation yet. Nonetheless, it seems that the -frame switch works better, but it has the same limitations as -screen (you can't get overlaying windows content). You can also try to minimize all open windows.
If that doesn't help, use xwd for correct captures, or grab your hitchhiking guide and get on google.
...But the fight against black boxes was not yet over!!!...
One day I noticed on one of my sites that a few of the wIMG screenshots were missing, there were getting queued but after reaching the end of the queue, they were just going back at the end of the queue... unfortunately it was not the first time I experienced something like this. After investigating the problem I found that the screenshots were not successfully taken with Imagemagick, it gave an error.
I had seen this error a few times when I first switched from "-window root" to "-frame" but it was just happening once in a while, but now it was happening every time, no screenshots were getting succesfully taken, even after I restarted my server the problem continued. So I was forced to go back to using "-window root" despite the risk of black boxes.
I was so scared that the black boxes would return that I upgraded the "part2_logs" to actually show the resulting pictures for each operation and kept and eye on the logs for a few days but no black boxes all the pictures were perfect.
A few days later I check again, I randomly open the latest log of monkey #9 and see that all his last pictures, for as far as I could see, had a black box on them, each of them... I checked another monkey and that one had no black boxes the images were fine! So since lately I made a modif where I can keep a VNC open on each of the monkeys to watch them work live (I VNC in the computer on which all those VNCs are opened next to each other) I checked the screen of monkey 9 and saw that on his screen he has the Firefox Crash report window open!!! and that window seems to coincide perfectly with the shape of the black box... The browser was openeing and closing and taking the shots but this stayed opened behind and caused the black boxes!! after closing it, they went away. #9's pic started being fine again...
So I found an article on how to disable that crash reporter, they suggest 1 method that is unclear and another which is simple but has a mention that it "may" not work anymore, so anyways I tried the second one and havent seen the crash report window so far, will keep an eye on it!
Can I disable Crash Reporter?
Mozilla Crash Reporter is enabled by default and there is no user interface to disable it. Advanced users who prefer the standard application error notifications (e.g., the Windows error dialog "<application> has encountered a problem and needs to close" and Event Viewer record) can disable the Mozilla Crash Reporter by adding a system environment variable named MOZ_CRASHREPORTER_DISABLE and setting its value to 1.   Note: Another method to disable the Mozilla Crash Reporter was to open the "application.ini" file in the installation directory, find the entry [Crash Reporter] Enabled=1 and change the 1 to 0 (repeating the edit if an application update replaces the file) but this may no longer work since the fix for bug 686466.