Richard,
Thanks for contacting me, and I'm sorry about your problem. But you have encountered an issue that I have not heard about before. It may take a few email messages to figure out what is happening.
My first guess would be that you are trying to use an image so large that the app (and your phone) cannot fit it all in memory. Sometimes displaying large images needs more memory than Android allows Custom Maps app to use. In that case Android kills the app and shows the "Custom Maps has stopped" dialog. If you are trying to use a very large jpg or png image, try making it a bit smaller using an image editor, and then try to create a map using the smaller image. If you are trying to use a PDF document with a very large page size, then you may have to convert the PDF to a jpg or png image using a different tool. Custom Maps assumes that any PDF page rendered at 200dpi will fit into the device memory, but if the PDF page holding the map is a really large one, the resulting image will be too large to handle.
If you create a smaller image, note that the current version of the app cannot handle spaces, special characters, and national characters (sorry about this). You should use only letters a-z, A-Z, digits 0-9, and dashes and underscores (- and _) in the image name.
If you are not using a very large image, I need more information so that I could try to make additional guesses on what might be the problem. If the map image that you are trying to use is not private (or you are not otherwise forbidden to send me a copy), I would like to request you to send me (not the user group) the map image file you are trying to use. That way I could try to reproduce the issue on my own phone, and it would be easier to figure out what is going wrong.