Perhaps Paint.NET is not the right tool for you. Have you investigated programs specifically designed for taking screenshots?
Windowclippings works well. You might try that.
Perhaps the files were stored in a different format only with the PNG extension.
Try renaming the files to different extensions to see if they can be opened.
I'd try PDN first.
You may wish to contact the vendor of the anti-virus software as they may want to inspect these files. If indeed they are false-positives, they'll want to correct their software.
You can do this yourself.
http://www.getpaint.net/doc/latest/Toolbar.html
Under Tools > Choose Defaults
Not going to happen.
Jpeg is a "lossy" file type. Paint.NET will alwasy default to a type that doesn't lose any information. That is why the default type for layered files is PDN and for single layered files is PNG.
The best way to select the text is to:
1. Place your text on a new, blank layer
2. Select the magic wand tool
3. Shift-Click the blank area outside of the text
4. Press Ctrl-I to invert the selection
Batch resize can not be done with a plugin. Instead of writing your own standalone batch resizer, why not just google up a free utility? Several exist already.
This only happens when you double-click on the layer name to open it. This is a function of Windows relating to double-clicking text. To work around this issue, open the layer property window from the menu.
Show us an image of what's not working and what you expect. If you expect it to work like another tool (Photoshop?) show us a screenshot of that tool working the way you expect.
You must be running the latest version of Paint.NET (which is 3.5.10) in order for most plugins to work. Upgrade your Paint.NET to the latest version and see if that helps.
With Paint.NET you can do what you want.
I think you may need to learn how to use layers. Start here: http://www.getpaint.net/doc/latest/WorkingWithLayers.html