poor quality export of still images from premiere This is the actual source image to a video track in Premiere (a PNG):

With no resizing whatsoever, and .flv output, this is a resultant frame from the exported video:

How could I improve the output quality, especially the text?
What settings should I use

I've tried to max out pretty much everything in the FLV export settings.
And now I moved to MP4 output with H.264 encoding, but still get poor quality stills.
Here are the MP4 Export settings, what should I change?


Solution
Inspired from the answers, turns out that have to set the source to Progressive when creating a New Sequence (instead of Fields: Upper)

this is why I did not get better results whatever my configuration once I had created a sequence with Fields set to Upper
and this is why the following settings for my next project, yielded good stills

although also using higher resolution, so not sure high that plays into the test results
H.264 / MP4 Export settings otherwise the same
That looks more like a 'field' from the video, rather than a frame -- jagged diagonals are the tell.

If your video is interlaced, you may only get half the vertical resolution in a still, unless you specifically set it to output a full frame. Check the export settings.