Might it be too small? What size are your JPG's? You are exporting at original size, and your watermark is only 50 pixels in one dimension (unless that's a typo).
Try exporting to an image size of 600 x 600, and set opacity to 100%. This should confirm whether your watermark is being used.
Then you can trouble-shoot either:
. the watermark is not being used, or
. the watermark is not giving me the result I expected.
You're welcome. Glad I guessed well.
Once I settled on a watermark I was comfortable using, I created versions at a range of regularly used sizes: 320, 640, and 1024. I assign the watermark and the export size to an Export Preset, and just select the Preset I want. My 1024 watermark works on images fit within a 1920 square. That's the biggest export I watermark.
It's a bit of set-up time after one has settled on a watermark to make different size versions, but -- as Jade Leary points out -- its the best way to use watermarks using Aperture alone (it is what is recommended in the User Manual).
I actually often combine my watermarked images with borders that have text on them. I do this with Border FX -- a free plug-in all of whose tools are worth getting to know. There is a watermark -- unobtrusive, the way I like it -- on the lower-left of this image superimposed on the image (not really visible at this resolution -- click and you can see it in the larger version), in addition to the title and date and name in the border.
Message was edited by: Kirby Krieger