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I dont know if this is relevant to what you are asking but I believe that the best paint program out there would be photoshop. It provide brushes from basic to advance to even custom if you know how to use the program enough. I mainly use photoshop for any paint related project. And as the means for getting the program I am sure that you could always ask a co-worker that you work in the field with Im sure its a pretty easy program to get a hands on. I think a majority of the internet knows how to or know of the program photoshop. So if you havent loooked into photoshop already you should take sometime to take a look at what it has to offer and its features.
Paint.NET is quite nice. My dad downloaded it and I've used it once or twice--it seems like an improvement on MSPaint (a rather large improvement, though, mind you.)
Now, of course, besides Paint.NET, everything basically boils down to two choices: the GIMP or Photoshop. If you had money, I'd definitely recommend Photoshop (it's the greatest!) If you lack money (which seems to be obvious from your post), then I'd suggest just downloading the GIMP. Since you already seem to have the GIMP, if you are still unsatisfied, perhaps you could try Paint.NET. But I do think that the GIMP has more features than Paint.NET, so it'd be rather pointless for you to do so.
You can't wield a big sword if it's too heavy for you. While Adobe Photoshop is capable of doing more, they're hardly features you as an amateur will scratch. It would be easier for you to work with a program like GIMP or Paintshop pro which are a bit easier to understand.
It doesn't matter how good or powerful something is if you struggle too hard to learn how to use it.
.rar is a archieve compression format like .zip. Not an image format.
Basically there is GIMP and Photoshop. If you can't or don't want to buy Photoshop, use GIMP. Now that the LWZ patent has expired in most countries, saving to .GIF shouldn't be a problem and JPEG has never been a problem with GIMP.
Now GIMP is NOT photoshop nor comes close to replacing PS. But it does argueable do something better. I prefer the lens flare effects from GIMP over Photoshop so I tend to use both. That being said, I use PS with various KODAK filters for restoring pictures every day. But then again that IS my business so...
I don't see why you would need another image editing program if you have The GIMP already. If you don't want to spend money for another program, The GIMP is probably the best one of them out there. It will need some getting use, but once you know where things are, it should be simple to use. From what I found, it supports many types of files already (editing and saving wise):
It would help if you explained which file format you need to save something in. Check out Download.com for suggestions. If that isn't the category you're looking for (above link), select another one and remember to sort by Free license. That way you don't have to pay $500-$700 for good software, you only see free ones.
I hate time-limited trials too, feature-disabled trials aren't too bad but I still prefer free and open-source.
I know yEd Graph Editor is good for creating SVG images.