To add further to what MikeF said, any serious photo editing software will allow you to select the degree of compression when re-saving a file in JPG format. This option takes various forms depending on the software, but it tends to be a "slider" that lets you select a range, such as [largest file]...[smallest file] or [least compression]...[most compression]. It may vary with the software, but the least compression (or largest file) option should result in little or no additional compression beyond that already done by the camera when the photo was saved.
Even so, it is always a good idea to carefully save the original files from your camera and to make no changes to those files. That leaves your source safe with maximum resolution and minimum compression. Any changes you make (such as shrinking [resampling] or cropping) should be saved using save as to avoid replacing the source file. Later, if you want to make some other change to the image, re-open the original file, edit it, and save another copy. In this way, all of your edited files are just one generation from the original.