Note that some information is lost in the JPEG compression process, but any good graphics program will give you options on how compressed you want your JPEG to be. Save your source photo as a PICT or TIFF if you wish to use it for purposes other than the web, since these formats are lossless. If you are ever undecided about whether to use a GIF or a JPEG, experiment a little and compare the results. The PICT is the format you will use the most when you are not making web graphics since it is the native MacOS format. PICT files can support millions of colors and are much larger than GIfs and JPEGs.
TIFF files are a common, widely supported format that also supports millions of colors.
If you intend to use a service bureau of any kind, TIFF is often a good choice. BMP is the native Windows file format. Good graphics programs allow for interconversion between common formats, but if you encounter any exotic file types, you may need to download Thorsten Lemke's Graphic Converter. This is a nuisance if you are making small image files for the web. On Scaling It is easier to scale a 72 dpi image to 300 dpi than to scale a 300 dpi image to 72 dpi.