Here is what I do.
I use IrfanView, a free download for Windows. It can easily provide all of the conversion and touch up I generally need.
1. If a color image is dark as shown on the screen I will increase the chroma a bit to bring out the contrast. For gray scale I may tweak it a bit for good contrast.
3. I size the image for the screen. For me that is 318 pixels by 445. I do not include headers. IrfanView can reduce an image proportionally so I just pick the width or height whichever is larger and let it resize as appropriate. (it can do a batch mode where is resizes everything is a folder if you are in a hurry).
3. Once I get the sizes right I look to where the images are to be portrayed an some need to be made small so that they merge properly with the text. I even may move them a bit to get good pagination effects.
4. I make sure dithering is off and then convert the image to gray scale. For JPG this will reduce the image to 256 levels of gray.
5. For image that are not jpeg I further reduce the image to 16 levels of gray.
To see this at work download my posted Alice in Wonderland. For inline style images download my posted English Bible History. Both are on the last page of the IMP posts.