You can use nearly any graphics software for taking measurements.
The Notes and Resources step of this instructable gives links to notable image measuring software. Three free choices are IrfanView, The GIMP, and ImageJ. All are easy to use for basic measurements.
For simple measuring of lengths, I prefer IrfanView graphics software.
Keep in mind that The GIMP is excellent and is my main image editor. It is worth your time to learn it as a free alternative to Photoshop.
ImageJ is a scientific and math miracle. Easy to use, but loaded with incredible features. Thanks to i'bler aman0311
for suggesting this.
IrfanView is Windows-only freeware. It is easy to load and use. It is kept up-to-date by its author Irfan Skiljan.
Be sure to download it from the official site IrfanView
or from a trusted site like C|Net
It installs quickly and easily. It's very light on resources, and is easy to learn.