Hello, Im working on Thematic and have the same problem with Safari.
It seems like the definition of width/height in the img-tag is in a strange way messing things up.
When I remove that information from the output, the images are rendered as they are with no stretching. If the ratio is different the width will also differ and look quite messy in a gallery.
A solution could be to crop the images to the same size before uploading, and remove the width/height information from the html-output.