I've been struggling with this for the past few hours, and the solution above didn't fully work for me, so this is just to share my experience of how I adapted it.

Using PDF Creator did give me a 300dpi image, which was great. The downside was that when I printed a chart from excel with it, the dimensions were all distorted (it was stretched to fit on an A4 page). I solved that by exporting my image to Powerpoint first (I couldn't find another solution), but then ran into another problem (which seems to come up however you create a document with PDF Creator, that my image is now sitting in the middle of a sheet of (virtual) paper and needs cropping.

After more cursing, I managed to solve that by downloading GIMP (open source image editing software) and cropping the image - it seems to still be 300dpi, so fingers crossed, the journal will accept it.

What a rigmarole!