I use to work for webs, and now I need to print a few tags. So I prepared the first file (120 mm x 40 mm, 300ppp), saved in JPEG, and... what did I get, when I opened it in Photoshop to check? A picture of 120 mm x 40 mm, that's OK... but no 300 ppp but only 72 
Therefore, measures in pixeles are just 341 x 114.
What's wrong? Thanks for your help!