Converting JPG to PNG is technically simple but often misunderstood. People convert hoping to "improve" the image quality, but that's not how it works. A JPG that's already been compressed has permanently lost some detail. Converting it to PNG preserves exactly what's left — no better, no worse — but the file will be significantly larger.
So when does JPG-to-PNG conversion actually make sense? When you need transparency (you'll need to remove the background first, then save as PNG). When you're going to edit the image further and want to avoid additional JPG compression losses on each save. Or when a specific tool or platform requires PNG format.
The fastest conversion method is a browser-based tool like fwip. Drop your JPG, get a PNG back. No software installation, no upload to external servers. On Mac, open the JPG in Preview and use File → Export → Format: PNG. On Windows, open in Paint and Save As → PNG.
For batch conversion, IrfanView (Windows, free) and XnConvert (cross-platform, free) can convert entire folders of JPGs to PNG in seconds. Just be aware that your total storage requirement will increase — PNG files are typically 3-10x larger than the equivalent JPG.