On the left side of your screen in Eclipse go to the Package Explorer
Open "src/main/resources" , then "assets.foundations.textures.items"
Right click on your mySword.png to bring up a menu.
Now in the menu find the words "Open With" then click "Other..."
Next in the Editor Selection, click External Programs, and then click browse.
After that, you will need find Paint.net in the "Open" window .
It's file location is one of the following:
in the folder click "PaintDotNet.exe" and then click "Open."
Now click the two check boxes underneath that say
"Use this editor for all 'mySword.png files" & "Use it for all *.png files" .
Then click "OK". This will make Eclipse open all your future textures in paintdotnet.