Whew! Anyway, once your files are all open within the same Preview window, click on any thumbnail from the sidebar and then press Command-A or choose Edit > Select All to, well, select all of them at once: (And also be aware that if your JPEGs or PNGs aren’t opening into Preview at all when you double-click them, you can adjust your settings for that, too.) Which is sure handy for batch conversion as we’re doing here, but it may not always be what you want! So just know you can come back to Preview’s preferences and change things whenever you’d like after doing your conversion. It won’t matter how many I do at once, they’ll all appear within the same window in Preview. If, however, I have “Open all files in one window” on instead, then I could double-click on the first JPEG to open it, then on the second, and so on. If I then double-click on any of those files, Preview will open with them all shown together in the same window, like so: To do that, I held down Command and clicked on those items on my Desktop. So with “Open groups of files in the same window” on, I have to be sure to select all of the image files I’d like to convert at once, like so: Clear as mud? Here’s what I mean by that. If you’ve got “Open groups of files in the same window” chosen as I do above, double-clicking on images will open them together only if they’re all selected at once if instead you pick “Open all files in one window,” then you can go through merrily double-clicking on any picture files you want, and they’ll all appear to be within the same working file in Preview.
When the Preferences window opens, click on the “Images” tab and make sure the radio button next to “When opening files” is set to one of the first two options (i.e., it can’t be on “Open each file in its own window”).