Try defining a limiting width (and optionally, height) parameter to the background like this:

myhtmlfile.htm (w.95,h.95)

You can substitutue any value you like eithyer in pixels (1-any value) or a percentage of the screen width (from .01 to .99).

See if that helps. Note that a style line in the HTML file like the following will help it blend in with the default colours that MediaLab uses:

<body style="font: 12pt arial; color: navy; background: #ccccff; margin: 0px">

With regard to randomization, there is no reason why that should be affected. I'd have to see the files.