tiny ppi editor

I had a picture representing an A4 page, I resized it with Microsoft picture manager (changed definition) to lower the size of the file, but in the process "picture manager" didn't change the ppi "pixel per inch" automatically to keep the picture to the size of an A4 page. So my other tools (an "all to pdf" converter) couldn't understand the right size of the picture and the output was a very small page.
You need softwares like irfan vue, xnvue, or photoshop to edit the ppi, which doesn't make sens to me.
So I made this tiny tool, with C#, to allow changing the ppi of a jpeg file very easily.