Artcarver,
The software I use for my carvings is called MeshCAM Art. It is really easy to use, once you gain a comprehension on how to use it. It will create tool paths from simple bitmap files such as JPG's, PNG's, BMP's etc. As for the feed rate used in a g-code file, it is not quite as difficult to change (WITHOUT altering the Z axis in Mach's Motor Tuning screen). In fact, you can edit the g-code file using a simple text editor (such as the Notepad program that comes with the Windows operating system). To change the feed rate using Notepad, simply open the g-code file desired, then click on Edit (at the top of the Notepad window), and select Find and type in F then press the Return or Enter key, Notepad will find every occurence of the letter F contained in the file (beginning with the first one it finds, it will stop and wait for you to type something, before continuing to the next F that it finds).
For example, if the feedrate of your current g-code file is 50, then the first (and possibly ONLY occurrence of the letter F contained in your g-code file) will say F50. To change to a different feedrate, simply highlight the 50, and type in your desired feed rate, then press the Enter or Return key. Make sure you save the file, after making this change. And then regenerate tool path in Mach3 (so that Mach will read the new feedrate).
For more info on using MeshCAM Art, please see my article in the Winter 2013 edition of Digital Machinist Magazine on Digital Sculpting. It goes into a lot more detail on the process of creating 3D g-code files using Corel DRAW, and MeshCAM Art. Hope this helps you, I know how confusing it all can be, when first starting out in CNC.