caliper

I have to make a rough measurement of a narrow width in some pictures. some experimentalists suggested that I made 20 random measures and then average. My problem is HOW to measure ... SuSE has an implemented graphic ruler. What are the units for such a ruler ? WHen I slide the mouse pointer on it I can see numbers changing frequently close to a text string "px". Shall I understand those are pixels ? or what else ? Does SuSE or any other Linux distribution have other measurement tools like a caliper or ... ??? Is this the ONLY availabe graphic tool for visual length estimates ? Thank you very much. MEM