Does anybody know of some good (relatively inexpensive) photo editing software that allows me to draw straight lines & irregular polygons.
All the graphics professionals will tell you to get Adobe Photoshop CS3 Extended. This will do exactly what you need and it will do it with precision. But it is also expensive and maybe a bit of overkill in terms of features. It can do a lot now, like measure lengths and area and take care of perspective transformations, likely more than you need.
Adobe Elements is the stripped down version and I think it still has vector lines. The feature is intended for graphic artists who need to draw vector based masks to make borders and odd (polygon, round, oval) shaped images. Adobe has announced an upgrade to Elements so if you buy now you'll be upgrading soon.
The other one to look at is Gimp. It is much more sophisticated than those other programs you listed. It is closer to Photoshop. Many people don't like it because it is not simple and has a learning curve about as steep as Photoshop. But that is the price you pay for all complex and capable programs. The Gimp certainly can do vector graphics. It lacks some features that Photoshop has that you may not need such as color management, 16-bit and deeper color and color spaces other than RBG (such as CMYK), But it can do vector based overlay and it's free. see
http://www.gimp.org You can also go to Amazon.com and look at some gimp books. These are a few to choose from if you don't like the the book that ships with Gimp as a PDF file.