Inkscape

Essentially all images throughout this site are .svg files (Scalable vector graphics) that I’v drawn in Inkscape.

This means you can zoom in as much as you want without the images getting blurry. If you just want to browse a pattern, you can open the image in a new tab to zoom and pan around. Use Inkscape, Adobe Illustrator, or any other vector graphics program to make arbitrary measurements in the image. The svg files are drawn to scale, so your program will be able to make measurements in physical units like centimeters or inches, not just arbitrary digital units like pixels.