The short answer is anyone!
What matters is sharing your concerns in an artistic fashion, so that others can hear of the Transgender Day of Remembrance , and events relevant to that. The numbers and composition of the participants varies each year and it is understandable that a participant from a previous year may or may not be able to contribute for the current year (for any number of reasons). New contributors (and returning ones) are welcome. The main requirement is that your remembrance artwork / image / video / text be accessible by web in general - that is, it shouldn't only be produced for this archive. You don't have to be a transgender person to participate, just appreciating or understanding the tragedy of the lives lost that are memorialized here is enough.
You don't only need a continuing webcomic to participate. Any suitable artistic/original image (photos, snapshots from virtual worlds, et cetera), comic, cartoon, video or text (poetry/prose/song, rather than a long article) will do, so long as it is published publicly (that is, where others can read it).
What's a suitable contribution?
To participate you need to create a visual artwork, a relevant snapshot or screen capture from a virtual world / online game / m.m.o., et cetera, showing participants observing the day or in-world/game memorial, a video, with/without prose/poetry relevant to the above, or stand-alone prose/poetry text...
...that appeared on a website (webcomic, blog, Deviant Art type site, Facebook, Google+, Live Journal et cetera) that is read by others. The subject of the archived work should be about either...
...the Transgender Day of Remembrance in general,
a specifific event memorialising the day,
or about a specific murder or suicide of a transgender person(s).
The above may seem a bit unweildy, but it gives a wide set of choices in creating a contribution. Once again, the important thing is publicising TDOR, TDOR events, or drawing awareness to trans murders and suicides.
Timeliness and Connecting
Contributions referring to TDOR in general or to an upcoming TDOR event are best publicised before the 20th November. That way when people read your contribution they might be able to attend a TDOR event. Contributions published late are no good for that. Creative works about a specifiic murder or suicide are not really constrained by this, obviously. In fact the custodian of the archive will often seek out works that become contributions after the fact. This archive is about memorialising trans murders and suicides, because both are often either misreported or go unnoticed at all.
Lastly, don't forget to link! You should at the very least link to the International Transgender Day of Remembrance site. If you know of any TDOR events in your area, it's a good idea to link to an information page listing that event (especially if you're atending). Note that neither of these links is shown in the archive, even the links were in the original, because by the time the archive version is uploaded, both events have happened.
If publishing to social media it's also helpful to add hashtags or keywords to your post/page to help others find your contribution. Go read the Social Media page for suggestions on this.