To scale drawing of lines/loci/etc

I often find myself in a position where I need to draw to scale lines & loci on a bitmap image.

It's relatively easy for me to define a scale in terms of, say, metres per pixel - so if I had photoshop, I'd just draw a line and then give it an appropriate outer glow in pixels, which would function as a locus around my line.

However, at work, I do not have photoshop. Are there any free programs which can do such actions? At the moment I'm using paint.net, and while I'm able to form loci, it's time consuming.

Any suggestions?

Thanks, B