Thanks for designing this plugin. I had a similar symptoms to what Denver R had, but I was able to discover my problem. The plugin fails reading the bitmap header when a 32-bit bitmap with alpha channel was generated (I used GIMP to do my bitmap conversion / save), but works fine with a 24-bit bitmap. You may wish to have the plugin specify that only 24-bit (or perhaps 16) is supported, as there are multiple bitmap formats. GIMP (and perhaps other tools) may default to the 32-bit bitmap.
When this occurs, the Ruby Console generates an error claiming the header error, but still progresses, allowing you to select a width (with a nil computed length). Then, upon selecting a depth, it throws another error, but still waits for you to select a depth, as if the plugin is hung, but SketchUp remains responsive.
Therefore, it may help to generate a pop-up error or simply advertise support for specific bitmap types. Thanks again for the plugin, nice job!