I use Corel Draw and trace the image to a vector outline.
Don't know if you know what vector is so I'll give a little explanation (as best as I know - not long ago it seems I was asking your question). Bitmaps, jpegs, and many other file formats are comprised of lots of pixels to make the image, this is no good for a plasma cutting file. A vector file is like a bunch of mathematical data which represents the picture, and vector files can be limitlessly resized without any loss of picture quality, unlike pixel based files.
Some programs like Corel Draw have an automatic trace utility built in but depending on the picture you are tracing this could work out OK or be a complete mess. At the end of the day you are trying to create cut lines / paths, so a very simple example of where auto tracing would work is a completely solid black circle on a white background. There's a distinct separation between the white and the black and the auto tracing will create a line at this path. Now compare that with different shades in a photo and the auto tracing can get really confused and to put simply probably won't give you anything representing a cut path.
Now you're into the world of manual tracing and you'll have to learn about nodes, curves, lines, etc in Corel Draw or what ever other program you're using. There's a good tutorial you can purchase from Macromonster on tracing in Corel Draw. Then there's a training course called Corel Draw X5 Unleashed. All money of course.
A free program is Inkscape but you'll have to do some Googling and asking on the forums for tutorials teaching you how to use it. From what I've heard it's downfall is that it exports horrendous DXF files comprised of lots of short straight lines to make up curves. Corel Draw is the same BUT you can purchase DXF Tools for Corel Draw which outputs the DXF file as nice curves and lines. Many people seem to purchase Corel Draw purely because of this add on program to give the nice DXF files. I also find training materials are much easier to come by for Coreldraw and a few users reckon Corel Draw is easier to learn.
I've finally managed to buy Coreldraw, DXF Tools, training materials, etc and well I like them, just get a bit sick of all the learning involved before the actual cut path is achieved.
Once you've got your DXF file (or SVG file or whatever) exported you'll need to put that in a CAM program to create your machine Gcode / cut path with its pierce points, kerf width, offset, lead-ins, etc. Sheetcam is a good one at a reasonable price.