I usually read manga from online readers, specifically MangaPanda.
Am I contributing to a mangaka's income by reading from such websites? Are they licensed to distribute them online?
The short answer is no. Most manga readers you find out there are all third-party aggregators (therefore not licensed to distribute the manga in any form) that take fan made scanlations and host them on their respective sites. They make money from the ad revenue they receive from the traffic other sites. In other words, they take the hard work of others (specifically, the work original authors and artist, along with the volunteer efforts of the scanlators) and profit without doing any work themselves.
There are aggregators out there such as Batoto that maintain a certain quality standard such as not resizing, downsampling, watermarking scanlations, or scrape content from other scanlators or sites. They also respect the wishes of scanlators that don't allow aggregators or have rules on delayed releases. Even in doing so they are not officially licensed to deliver their content, while they do try to maintain a standard of quality, none of this benefits the manga authors.
However, there are various officially licensed distributors for English translated manga. Here are a couple of the more well-known ones:
However while this might not directly influence revenue to the manga author. Making the manga more widely accessible to the public can help give some notoriety to a particular series. Hype and notoriety about a particular series can indirectly boost sales of the series. However it comes down to the efforts of scanlators and fans to help boost revenue to a manga author.
For an example, the users of 4chan helped boost the sales of WataMote and save it from the possibility of being canceled. The author personally thanked 4chan on Twitter for their efforts and those of the scanlators.
Even if you read manga from these online aggregates, you can still support the authors by purchasing their manga when they are collected in volume form, from such sites such as Amazon Japan or Honto.jp.