Paint.NET :: Images That Changed To Transparent Background Showing Up Black

I just learned how to make the background of my images transparent and am learning how to use layers. I am trying to save my pictures to picasa and photoshop so they are backed up and I can easily access them from any computer. I noticed that some of the pics are showing up all black. It's like the colors are inverted almost. I can kinda see the image in white, but mostly the whole thing is black. I noticed the same thing happened when I tried to open some of the images in paint.

I checked to see if they are all the same file type and they are not. Some are gif and some are png.

Also, some of my images won't open in windows photo gallery. They open in a blank web page. I originally go to my pictures and find them there, but when I click on them they open a page in IE instead of windows photo gallery so I can't crop them or anything.

I am trying to work on a project where I print silhouettes or simple images on vintage sheet music to use as art. I need to be able to take them to a print shop to have this done and I can't do that if the images show up black or won't open in a regular program.

I am attaching a pic that saved black like I talked about and a pic of what it is supposed to look like.