To do a proper job of making a board, you'll need a few tools. Sure, you could try to draw a board pixel by pixel using Microsoft Paint, but you'll end up wanting to kill yourself, and odds are good that your board will not look very good.
At a minimum, you'll need a decent graphics editor. You'll also probably make use of a good online image search engine, some fonts, some stock imagery, and depending on your image editor some powerful plug-ins. This tutorial tells you about a few of these tools and shows you where you can get them.
Remember to always be careful when downloading any software, WarGear is not responsible for applications downloaded outside of WarGear.net
Paint.NET is probably the most powerful and fullest featured free graphics editor available. The 'layers' functions will be one of your most important tools in making beautiful, functional fill-mode boards. Its abilities are expandable by user written plug-ins.
You'll also be wanting plug-ins. Plug-ins are pre-programmed image editing effects that make Paint.NET incredibly more powerful. These include tools to automatically outline shapes, cleverly blur background images to save memory with minimal distortion, to adjust colors and transparency, and more. You can find these in a forum at: Paint.NET Plug-ins
Fonts add a lot of character to the text you may use in your board image. Also, using a Dingbat font can save a lot of time if your board uses some sort of icon to denote a territory.
Dafont.com is a great site for free fonts. Be sure to read the license granted by the author of each font. There are plenty of other options for pay and free fonts which you can find via a web search.
You'll be wanting to use this to find images to edit or trace for your boards, in case you don't intend to draw one totally from scratch. You can use the web search to browse through dozens of interesting or strange boards blogs and websites. You can also use an image search to look directly for images you can turn into fun boards for Wargear.
Useful search engines include Bing which provides a great breadth and depth of results on a single scrollable page (no "Next, Next, Next..." pages to browse through 20 or so images at a time). It also allows you to quickly sort your results for B&W vs. Color, photo vs. illustration, and more. Google tends to yield broader results based on a single search term, but has few tools to further sort those results and seems to lack some of the diversity of results that Bing provides. As with fonts, be sure to check for copyrights and other usage licenses for your images if you find them out on the world wide webz.
Wikimedia is an online source for free, public domain images. If you want to be absolutely sure you don't need to worry about copyrights or usage, look for your images here. If there are license issues with an image served on Wikimedia, they make it very clear what you can and cannot do with an image.
Mayang has tons of stock imagery and textures to use for background and frames on your boards. Textures come in plenty handy when you want to add some character to a board image.