exporting pictures with a PNG watermark to the exact ratio of 1:1 is impossible! that mean loss of resolution and quality of the watermark I believe this is a BASIC function. Also the border distance trim for the image watermark, have too big steps... thanks
There is a partial workaround (at least in LR4, I'm not sure about 3.x). Assuming that your export dimensions are constant, you can place your sized watermark on a transparent background with a canvas size the same as your export dimensions. This can be saved as a PNG file to preserve transparency and in Watermark Effects, the option Size: Fit should be selected.