The 72 last silhouettes submitted to PhyloPic as of today ca. 11am Canberra time break down into (not necessarily monophyletic) groups as follows:
11 non-avian dinosaurs
28 other mammals
8 land plants (all submitted by me)
4 other organisms
These are known species numbers for the various groups:
>1,000 non-avian dinosaurs known from fossils
ca. 10,000 birds
ca. 450 primates
ca. 5,000 other mammals
ca. 10,000 reptiles
ca. 33,000 fish
>1,000,000 arthropods (presumably a vast under-estimate)
>300,000 land plants
ca. 1,800 brown algae kelp species?
Other groups not mentioned here would include >25,000 nematodes alone, with 1,000,000 species estimated to exist, and of course molluscs, diatoms, bacteria and so much more.
In other words, even leaving aside the 'other organisms', relative to their species number birds are over-represented by a factor of four among recent submissions, non-avian dinosaurs by a factor of 220, primates by a factor of 320. Unsurprisingly then, arthropods and plants are vastly under-represented. And of course the database was already full of monkey and dinosaur silhouettes before those last 72 submissions.
This is par for the course; the same kind of bias is why you can get a Nature paper for discovering a new species of dinosaur or sufficiently cuddly mammal but not for discovering a new species of sedges.
Still, do contributors assume that bryologists and nematologists will never need to illustrate a phylogenetic tree figure? Just wondering.