For Revolution, RamdomChunk should be called like this (note the double brackets at both ends and no ! at the end):
[[!RandomChunk? &chunks=`chunk1,chunk2`]]
Start by creating your chunks with just a text message: "Chunk1" "Chunk2" etc. The chunk names are arbitrary, but they’re case-sensitive. Be sure you’re using back-ticks, not single quotes around the list of chunks and that there are no spaces in the chunk list.
Once you have those messages showing up, change the text message in each chunk to a regular HTML <img> tag.
I should mention that I haven’t tried this with Revo, though the snippet claims to work in 2.0.