You have to extract the textures from your .jar first, like others have said. Just highlight all the texture files and drop them in a new, unzipped folder on your desktop. Opening files and trying to change them while in a .jar or a zipped folder usually doesn't work.

Listen, you're just opening in in the 7 zip program, you're just viewing the files, you have to extract them first. Right now they are a .7zip file, no program, other than 7zip can open them.

Extract it and save it somewhere else then try. Otherwise take note of where you saved it to and use the open function in paint.net

Right click on it and click on open with and select a program that is able to paint a block...that's just a photo viewer program, your on not a photo edit program...if you do have one then you can get paint.net or gimp.there free but what I use is photoshop elements 2 but it cost money...I do know gimp is better then paint.net but if your a beginner I suggest sticking with paint.net.hope this helps. Also for more of safety purpose I suggest running a virus check on paint.net or gimp if you get one of those because I never used them but I heard it was one of the best free programs...my friend uses paint.net so you can also trust that a little bit more.

You have to right click the .zip folder, and unzip it, you can just open it the .zip folder and try to edit it.