I’ve found an annoying bug in Lion’s Preview. If you open an image and select “Export” to save it as a new file format, you must change the file’s name after you select your target format from the drop-down menu. If you don’t, your custom name will revert to the default when you choose the new format.
For example, let’s say I take a screenshot. My Mac names it “Screen shot 2011-09-20 at 10.57.50 PM.” Next, I open the image in Preview and select “Export” from the File Menu. Then I change the name to something like “safari_screenshot,” and finally select “JPEG” from the format drop-down menu. The name will then revert to “Screen shot 2011-09-20 at 10.57.50 PM.” This was not the case with Snow Leopard.
I hope this gets fixed soon, as I use Preview a lot for simple image editing.