Jpg files are compressed meaning data is lost and only 256 levels of lightness is used. The lost data is often not meaningful such as in shadows or clouds. But when really interested in best quality, scans should be tiff (tagged image file) which are raw uncompressed data.
This is not quite true. Jpeg images store many more than 256 colors (I believe 24 bit color, which is 16.7M colors). However, it is true that they use lossy compression. If you turn up the compression too far, you'll get blocky images (the "jaggies").
I agree that Jpeg is not the format to use for prints, Tiff is much better. Tiff is what I use when I need a digital print. Tiffs is a nice, long-standardized file format, which means anybody can handle it. Tiff files
can be compressed losslessly, meaning image quality will not suffer from the compression, but I don't know whether the 1-hour labs (or any other labs, for that matter) will be able to handle compressed Tiffs. I always just burn them onto a CD uncompressed... big file, but no loss, and no worries about files the lab can't process.
I do try to find out what physical resolution the lab prints at, so I can make the file exactly the size it needs to be to print, pixel-for-dot. For instance, if the lab's printer works at 300 dpi, and I want and 8X10 image, I'll make the image 2400X3000 pixels.
All of this is irrelevant, however, since you've said the images are in Jpeg format. Just burn them to a CD and take them down to the lab. They'll be able to handle them just fine. In the future, try to save Tiffs (or other lossless format) for prints, and Jpegs for uses like posting here on the forums or on other websites. I don't find Jpegs useful for much else.