Since Rapaire has inticated he intends to work in Photoshop or Photoshop elements, the "lossless" format of choice is the Photoshop "native" .psd. It's a perfectly satisfactory format for archiving, and gives about the same filesize as a .bmp, but with a lot more retained information. (A file with many layers can be saved as .psd, and recovered with the layers still intact and separated. Saving as a .bmp or .tif nearly always "flattens" the layers.)

The .tif format actually comes in at least 7 different "flavors" in the standard, and many programs (Adobe is notorious) apply additional "features" so that the .tif format is not predictably "portable" between programs. (A .tif produced in Adobe Illustrator may not be directly openable even in other Adobe graphics programs. You often have to intall an import filter and perform an "import AI .tif".)

If I recall, the .png format is a Kodak thing, and while quite a few "snapshot editing" programs will handle it, it's not directly usable for much except printing snapshots from Kodak licensed print programs. (I'd have to check notes to be sure that's the format I've run into quite a few problems with.)

The .gif format is "sort of" lossless, but isn't appropriate for color photographs. It can only display 7 (?) colors, and each .gif file is supposed to (but sometimes doesn't) include a statement that says which colors to use. It's okay for b/w or simple color lineart, but is not a very good choice for anything with continuous tone variations. It's lossless, in that you can re-save it repeatedly without losing anything, but you have to throw away a lot of information to make it the first time. As a side "feature," a .gif that indexes to colors significantly different than the inks in your personal "7-color photo printer" may make really lousy prints, and it's very difficult to correct for.

The key is to save your original scan - or whatever source you have, and then never save it again and never make changes directly to it. You can copy it as many times as you want. Work on a copy, in a lossless format, and when done archive the lossless "edited master" and make whatever other files you need from it (or from a "working copy").

The .jpg format can be compressed or not, and if you use a "high resolution .jpg" there's very little, if any, loss of information. Photoshop Elements allows you to select 10 separate levels of compression for .jpg, each in 3 different .jpg "modes." At the 2 or 3 highest .jpg resolutions (minimum compression) a .jpg file is likely to be about 30% larger than the uncompressed bitmap or .psd. Most snapshot editing programs allow only compressed .jpg saves, and generally compress like the 3 or 4 "lowest resolution" (most compression) settings in PE2. These can be extremely lossy if you save repeatedly.

Many professional photographers do use .jpg format for routine touchups; but they work from a copy of the archived original, open the copy once and make all the changes, and then save back to a high res .jpg. Lower resolution (lossy compressed) .jpg files can then be made from their edited high res file.

Most .jpg files you find on the web are compressed at one of the 3 or 4 lowest quality settings (highest compression) available in Photoshop or Photoshop Elements, often with resizing and reduction in pixel density before the compression and save. Many of the photo editing programs available allow little choice about how to save, and generally use these compression levles. Editing in another (lossless) format is a necessary measure with them. For something as "labor intensive" as restoration of old photos, using a "re-saveable" format is definitely in order, and you would normally save/archive the "final" version in the same format in which you worked.

John