From one slightly mature lady to another: Welcome to the forum!
Yes, I agree! Always save an SVG version of your file, if you think you might ever want to edit the image in the future. If you don't save an SVG version, you might not ever be able to edit it, without starting over from the beginning.
When you received that particular error message, what had you been trying to do? I've only seen that kind of error message a couple of times. And mostly it's when I've tried to change the format by simply saving the image with a different extension. So that could be the reason why the DXF didn't work, if you didn't take the proper steps to create it. If you imported an image, and simply used Save As DXF, that will not create the vector output that you need. That could be why it didn't work.
If you are converting existing raster images (PNG, JPG, GIF, etc.) to vector (SVG, EPS, DXF, and others) I've written a series of tutorials. Depending on your particular goals some of these might help:
http://forum.inkscapecommunity.com/index.php?action=articles;cat=6For help with making files ready for different kinds of cutting machines (I mean the home/craft kind of cutters) you can try this other forum (kind of like our adopted sister forum):
http://www.inkscapecuttingdesign.com/smf/index.php. Personally, I might not be able to answer your questions there. The other members there are very knowledgeable and experienced, but don't visit very often. However, you can search while you're waiting for them. And I can always take a swing at the question, if it's urgent.
For other kinds of cutters, such as laser cutters or CNC machines (gcode), the only way to get support is through the forum shown on the extension's Help tab. But it sounds like you're talking about home/craft cutters.
Also, various sites where you might be considering to sell your images, might have specific requirements for the images they will accept. So you'll always want to read their terms of service and various license issues. And if you want to copyright your images, that's a whole other story, which needs some research.
Please feel free to ask for clarification, if you don't understand something