.png instead of .jpg

You should have a choice if you are using a mac. At bottom of your save window, there is a dropdown list of choices which include PNG and JPEG.

To preselect all your saves, go to Preferences>Advanced, and select "Drag and Paste Format" in middle of page, and then select the format you want to save with automatically.  

Klaus, If you want to use JPG, go ahead. But if you're taking screenshots, PNG is the better choice.

PNG is not lossy and will not cause any degradation of black characters on the white background. Plus, a PNG will be much smaller than a good quality JPG. What I mean by that is the only way to have clean text with a JPG is to use extremely small compression, ergo, larger sized images. To get the size of a PNG, you need high compression and that will cause a lot of degradation to the text.

What kind of image capturing are you doing that you want JPG?