A) This brings up a couple of misconceptions about saving image files. The first is how saving or resaving a file in a different format works. Let's take an example of a simple file shot with a digital camera and recorded in a high-resolution JPEG format. I'll call the file image.jpg.
First, I'll open the file in Photoshop. Regardless of the name of the file, it's now sitting in Photoshop in Photoshop's native file format. Something similar happens in any image-processing program. It can only edit in its native format.
I make some adjustments. Next, I decide to save the file, so I'll save it first in the native image format. At this point, I have to tell Photoshop to save it in something other than its original file type, even though it's in the native file format while being worked on. I go to File > Save As and select .psd and save the file as a copy. Now, I end up with a file on my hard drive called image.psd.
I also want to save this file as a TIFF file so that I can put it on a CD and send it in for publication. With the image still open in my image editor, I select Save As and choose TIFF as the file format. The new TIFF file is created on my hard drive, but the actual file open in Photoshop is still in its native file format because, again, an image-processing program can only make adjustments to its own file format.
Next, to send the image via e-mail to friends, I select Save As and this time choose JPEG. I could continue doing this, saving GIFs, PNGs, TARGAs, PICTs, BMPs and all sorts of other file formats, but realize that I'm still working from the native file format (which is the same as the image.psd file). When you use Save As, a new file is created, but the open image isn't affected.
Ultimately, it makes no difference in what order you save files from an open file in your image processor. It also doesn't matter if the files are compressed or uncompressed because as long as the file remains open, it's still at work in the native file format. It's only when the file is closed that you could have problems, but even then, it would be with compressed files, such as JPEG, not uncompressed files such as the native file format or TIFF. If you closed a file to a JPEG format, then reopened it and saved it again as a TIFF, you'd lose quality because of the compression changes.