Inpaint
Sometimes, you have a perfect picture, except for one part of it that came out wrong. For those situations, Inpaint is the perfect tool. It allows you to select just specific portions of an image, and then replace only those portions with NovelAI Image Generation.
You can access this handy tool above any generated image, or straight from the Image2Image UI
Goose Tip: You don't need to generate an AI image to use as a base for Inpaint. You can use it on any uploaded image for the Image2Image tool.
Masking
When you access this tool, the first thing you'll be greeted with is the Inpaint Canvas.
It is similar to the usual Image2Image Canvas, but with a completely different usage.
Instead of adding strokes of color to the image, you'll be drawing one or more blue-tinted selection areas over the image, called a Mask, that will determine what portions of the image you want redone.
Anything that is not marked by the Mask, will remain the same.
Goose Tip: Careful if drawing the Mask's edge too close to what you don't want changed, as little pixels might sometimes "still be seen" by the AI. If you feel like stuff from outside the Masked area is "leaking" into it, try expanding the Mask.
Generation
After you're done Masking your image, you can then proceed with the usual Image Generation process of prompting and picking options.
You can prompt for what's already in the image if you just want to reimagine it, or prompt for completely new things if you want to add details.
When you press the Generate Image button, you will get image generations that only apply the prompt to the Masked area.
Goose Tip: Even if there are multiple imperfect parts in one image, it's easier to fix them one by one.
• By Masking each part at a time, not only there's only one thing that can go wrong per generation attempt, you can also prompt for what you want of each part more precisely.
• Alternatively, if you're generating with more than one Masked area at once, you can always restart the Inpaint process with a newly generated image that fixed at least one of the issues, potentially saving generation attempts.
Overlay Original Image
Because of how it works, when generating images with Inpaint, small details of the unmasked parts of the base image may slightly change.
This setting makes it so the untouched unmasked portion base image gets re-applied over the final image.
While this guarantees that the unmasked parts of the original image stay a 100% untouched, it may also cause visual seams around the area where the Mask was drawn.
An example where noticeable seaming was caused by the Overlay Original Image setting.
Focused Inpainting
The Focused Inpainting button next to the brush and eraser tools allows you to select a region of your image with a rectangular selection. Then you can either paint your inpainting mask as usual inside this selection box or leave it empty, in which case the whole content of the box will be inpainted.
The key difference with regular inpating is that, before inpainting, this section gets upscaled to approximately one megapixel (the size of the selection is limited to a smaller size). This allows the inpainting process to add more details at a higher resolution.
Additionally, if you are an Opus subscriber, Focused Inpainting also lets you inpaint regions on large images at zero 
Anlas cost.
The Minimum Context Area slider, positioned where the brush size slider is for the regular inpainting brush, allows you to adjust the context around the selected area, which will not get inpainted and allows the model to see the surroundings of your selected region. This context region is marked with the translucent, red border inside the selection box.
Focused Inpainting may sound a little complicated, but in practice, it is quite easy to use. If you would like to improve the face details of a character in your image, you just press the selection tool button and put the selection rectangle around the character’s face. Generate, and the level of detail should improve.
Advanced: Outpainting
With a little ingenuity, it's possible to use the Inpaint to perform an Outpaint generation. By uploading a picture with an empty space added to where you want it to be expanded, and masking that empty space, you can essentially perform Inpainting outside the base image.
An example of adding empty space to the source image, and then prompting for high-heels.
Goose Tip: You can use NovelAI's Canvas' Resize Canvas feature to create empty spaces in images that are already selected for Image2Image. Don't forget the "Crop to closest valid size" button!