I have done this likert scale questionnaire and some questions are allowed to be skipped. In this case, some likert scale questions remain unanswered. When analyzing data, can I assign a score of 0 to those questions?
I don't have any expertise in the area of Likert scales and other questionnaire and survey concepts, but it seems like that would be distorting your data. Here, look:
If 100 people took the survey, and 90 of them didn't answer the fourth question, and you assigned "Neutral" by default, then it would appear in your data that most people are just okay uploading images to Wikipedia. But this might hide the fact that 90 people don't use upload images, and so the question in actually doesn't apply to them. Consequently, the answer of the other 10 people who do upload images will seem now to be outliers if they're not neutral, and you'll be drawing in inaccurate conclusion from your data.
If you assign a score, you are saying that the respondent answered the question. Furthermore, you are making inferences about the intent behind the respondent's non-answer by assigning it a score. When analyzing the data, you should only analyze the data that you have. Analyze the data using the responses that you have. In reporting the data, specify how many respondents did not answer the question.
If you have a question where many respondents skipped it, you should consider what it was about your survey that made them skip the question. Was the survey was too long and people were looking for things to skip? Was the survey question not applicable to them? This will help you improve your survey design for the future.