FFMPEG Quality & Thumbnail

Hi Guys,

I have a 2 part question.

#1. I have a script that works fine to convert uploaded video to an flv file using ffmpeg but the quality isn't great. I have tried increasing the bitrate from the default of 200kb/s to 700kb/s which seems to make the converted video much worse. What options should i use to get a better quality video output?

Here is the line of code im using:
$output = exec ("ffmpeg -i source.ext -s qvga -ab 128 -y output.flv);

#2. I have another line of code that then creates a thumbnail from the converted video but what i would like is the code to take a thumbnail from the central location of the video rather than just the start. How would i do that without knowing the length of the video to divide it?

Here is the current thumbnail code:
$output2 = exec ("ffmpeg -y -i output.flv -r 3.0 -sameq -f image2 -an output.jpg");

Thanks.