I didn't get some of the characters, but the ones I did get, starting from the bottom and working counter-clockwise:
-Gogeta (Goku + Vegeta)
-Sakuling (Sakuya + Meiling)
-Cirdre (Cirno + Flandre)
-Bardock
-Remilia Scarlet
-Kirisame Marisa
-Gohan??? (Wasn't quite sure)
-Kazami Yuuka (Majin version)
-???
-Broly
-???
-???
-???
-Hakurei Miko
-Mima
-Hoshiguma Yuugi (??? version)
-Hakurei Reimu
-Piccolo
-Konpaku Youmu
-??? (Might be Master Roshi with a mask on???)

The one you thought was Gohan is actually Goku. The top right one is Super Saiyan Turles. If you notice he's holding the tree of might fruit. The one above Goku is Hatchiyack, a villian from the Raging Blast 2 game. The one to Mima's right appears to be one of the new villians from the new DBZ movie. I don't know his name. Yuugi looks like she's been turned into the Mecha cooler army. The bottom left one is Grandpa Gohan, way back from Dragon Ball. As for the very center top one, I'm not sure.

Is that Ozotto who has three eyes in the very center top? If they don't know Ozotto before, he's from DBZ arcade game known as Dragon Ball Z: V.R.V.S: http://dragonball.wikia.com/wiki/Ozotto

Also, since it wasn't mentioned above, judging by the lines on Broly's face, he's been body-snatched by Baby/Bebi/however-the-hell-you-romanize-it. Vegeta's face looked the same when he got possessed.

Also, while its true that Hatchiyak appears in Raging Blast 2, he originally appeared in the DBZ OVA "The Plan to Destroy the Saiyans", which was made for a Japan-only game years ago.

Popo is the strongest! everyone in this picture will scream like a little bitch if they fight him.