There are some brilliant aerial photographs of Margate in the 1920s here:
http://www.britainfromabove.org.uk/asea ... ch=margate
Unfortunately there don't seem to be any pictures of Dreamland. The rest of the collection on that website is well worth exploring; there are over 1 million pictures including some great ones of other amusement parks like this one:
http://www.britainfromabove.org.uk/imag ... ool&ref=17