Help

If you are unfamiliar with this genre of game, the following Level hints may be useful:

Level 1: A button! Don't hesitate to click it. Then carefully follow the given instructions. Remember Google is always your friend.
Level 2: Do you know how to use image editing software? Use it to reveal the dark. Again ask your friend if you don't understand the things you see.
Level 3: What does the image name mean? Apply it to the equations. Can you see the pattern? It is about subtraction.
Level 4: Is the image name strange? Change it to get more images. Anagram solver is your friend when you get a bunch of letters.
Level 5: Do you know how to check the properties of an image? Answer the thing you see. What cipher is it?
Level 6: You can now go on your own. You can do it! Don't cheat.

Here are some general tips:

If you think you get an answer, put it in the address bar by changing the page name. For example, if the current page is abc.htm and your answer is def, change it to def.htm. Don't use capital letters or spaces unless specified.
Some level requires authentication as an answer. Answers for those levels are in two words, one for the username and one for the password. Click the image to open the authentication box.
Always check the title and source code. There are valuable hints.
If you get a bunch of seemingly useless letters, use anagram solver to form a word.
Save your progress using String. If possible, write a log of your progress. They may be useful in the future...
Sometimes external programs are required to solve a level. A list of recommended software is given below.
Internet searching may be required to solve a level. It highly depends on your own knowledge. Therefore to prevent potential confusion, no explicit hints are given for such requirement.
Most importantly, don't cheat. You may (and mostly will) get stuck on a level, but don't give up. Take a rest and try again. Solving a hard puzzle after long time of frustration is really the apex of satisfaction.

The following software or websites may be useful: