...I did try stretching the forums version and it looked crap...
Try using
Image Analyzer (freeware), it has an enlarging filter called weiner, which works very well. It also has a plugin that uses fractals to enlarge images. It also has a very effective "adaptive smooth/sharpen filter" to clear up fuzzy images without making them look all blocky and stuff like regular sharpen filters do.
I started with the forum logo and enlarged it to 4x it's original size:
This one was done with just a standard enlarging filter (bicubic to be specific). The text came out nice, but the icon looks like crap.
This one was enlarged with the weiner=4 filter, then sharpened. The icon looks pretty good, but the text is rather off.
Now the fractal filter only enlarges x2, so to get x4 you have to either use it twice or use it and then a different filter.
Fractal enlarged x2, then bilinear enlarged to final size (sharpened).
Fractal x2, then weiner=4 to final size(not sharpened).
Fractal x2, then fractal x2 again (not sharpened).
Fractal x2, then fractal x2 again (sharpened)
If you took the icon from any of the fractal pics and the text from the bicubic pic, you'd have a pretty good looking enlarged image.
Image Analyzer also has a so-called "lossy png" option. The resulting PNG is actually a normal, lossless PNG, but the program runs a lossy filter on it before saving. It's not really useful in it's current state, but it does make for some interesting stylized effects:
...of course the simplest thing is to take the logo from the homepage and blow that up to whatever size you want.
http://allspark.net/cypherswipe/XNlogo-large2.png
Another handy little feature of Image Analyzer that I'd love to see added to Xnview is the color quantizer. This tool helps you to reduce the colors in the image in such a way so as to give you a much better quality/filesize ratio than normal color reduction methods. (First you use the quantizer on a true color image, then reduce it down to 256 or less.)