C-R-A-P uses Javascript to resize and crop your photos within your browser. All codes are executed within your computer without uploading any photo to a server.
Because of the natural of Javascript, all codes can be reviewed by another programmer easily and there is no way we can hide any malicious codes in C-R-A-P.
When an image is cropped to meet your desired dimension or aspect ratio, some parts of the image will be cropped out. The tricky part is to know which part to crop off and which part to retain. Auto focal detection helps to identify which part of the image is important.
Auto focal detection uses a brilliant Javascript library called "smartcrop". Generally, the important part of an image has more lines and curves than the background. From data science point of view, the "messy" region contains more data/information. CRAP uses smartcrop to analyse your photo and guesses which part is "messy" and retain that part and crop out the quieter surroundings.
By default, the browser doesn't allow a web page to save multiple files. The first time CRAP tries to save multiple files the browser will alert you. If you accidentally disallowed it, you can change this in your browser's settings page. Alternatively, you can use the save as zip file function.
You can easily find out how to reset your browser by searching keywords like "allow chrome download multiple files" or "allow firefox download multiple files"
C-R-A-P looks for xxxx's in the name and replace them with numbers. For example, my-photo-xxx will rename photos into my-photo-001, my-photo-002, my-photo-003 etc. You can put as many x's as you like.
Generally I recommend people to use 80%. You should never use more than 90% unless you have a good reason.
I assume the images resized are meant for online use. To improve the loading time and save bandwidth for mobile phone users, you should try to keep your images as small as possible. Of cause the photos shouldn't be so pixelated and they affect the aesthetic of your website. You can try out different qualities and find the optimal number.
Pixelation happends when you up-size your image and there isn't a good way to avoid this.