Normally jpeg are good enough. Jpeg is a file compression format. Infact it will compensate the image quality for compression.
Thus, it's still not the best format to save your digital work in. The best to use however is TIFF format. The is the highest quality you can save you work on. Quality of image is also dependant on how you resize your image, the scanning source, the settings you use to scan, the file format you choose to use in your digital camera, the resolution of your camera. All these will factor into the results of the digital image.
If you are sending it for printing, I will suggest you to have 2 versions of format. If permissable by the printer's equipment, the best choice would be to use TIFF format.
Yeap i truly agree JPEG is wat it termed as a lossy compression, therefore it will try to figure out what is important to the image and then save it (basically turn the colors to the freq domain and remove the higher frequency component) due to the nature of JPEG the moment u open the picture and then save it the file size will become smaller due to a bit of the image is lost due to compression. If u keep saving the same jpeg picture the quality will deteriorate even though u put at max compression.
Therefore my advice is to use a lossless format like TIFF to save ur originals and then convert to jpeg. png and targa are also good formats. but beware file size maybe quite big.
1. When you crop and save, you are recompressing the file. This ratio might be smaller than the original, which is why if you crop then resave, you might end up with a bigger file.
2. Don't bother about the PPI. Doesn't matter if it's 1, 300, or 4000. It varies with print size.
3. Don't bother sending TIFF. Even at 8 x 12", you are not going to see a difference between printing from a TIFF or low compression/high quality JPEG. But if you need to work on the file, save the intermediates in TIFF. Save the final as TIFF. Save a JPEG then send the JPEG to print.
4. The less you worry, the better. Crop, save, send.
4a. Better still, go to a good lab, then ask them for either their special "DI" paper, or ask them to do a "FIT" which will print full frame.