The quality checkers, and not the editors, are the ones that save the images to png. So you can completely skip this section. ^^
However, PNGs work by limiting the colors, which means color images should be saved to jpgs, anywhere from 60-80% quality. To save the color image, go to File>Save for Web... and use the following settings:
High is 60%, and Very High is 80%. You'll need to use Very High if the image has a lot of red, since red tends to pixelate easier.
The best settings (to create the best quality image in the smallest filesize) were posted by Ookla (from Toriyama's World). Go to the save for web panel, and select PNG-8 128 Dithered from the drop down menu. Then change the following: Change Selective to Adaptive, uncheck the transparency box, set Colors to 17, and set Dither to 100%. Then click the arrow near where it now says [Unnamed], and select Save Settings... . Save the settings as whatever you like -- and select this setting for every grayscale manga scan you save. (see all this graphically)
Now, if you're particular about filesize, the filesize on these pngs can be compressed by 3-5% (for Photoshop CS) or 10-15% (for Photoshop 7). For Photoshop 7, this will usually chop off 1~2mb from your final zip. There are two PC programs which will do this (in case one doesn't run on your machine), with instructions as follows: (neither program belongs to me, and I've copied these specifics from people far smarter than I)
Unzip optipng to an easy directory to locate. For example, C:\optipng\.
Place all your pngs into C:\optipng\
Again, optiPNG needs to be run through DOS. Go to your windows Start Menu>Programs>Accessories>Command Prompt.
This will open up a black box. Type "cd C:\optipng\", without the quotes, to enter your folder. If you put the optipng folder elsewhere, you'll have to change that command ^^
Type optipng.exe. You'll get spammed with text in the command box. That's a good thing ^^
Next, type in this command line: optipng -k -zc9 -zm8 -zs0,1 -f0 *.png
After a few moments, when the text stops scrolling across the screen, all your PNGs will be crushed and in the same folder. Your original PNGs will still be there, but will have a .bak extension. You can delete the .bak files. Zip up the crushed pngs, and you're done ^^
The last section of this guide showcases a few tips and tricks on how to save time editing.