Easy answer, start with a larger picture!
The FSM forum software shrinks pics to fit the space, and clicking on images will open them up to their "full" size, which could mean the same size as the posted image.
Photobucket can reduce images, rotate, flip, color correct, paint, texturize, distort, add thought balloons, but it can't take a small image and make it bigger!
You, on the other hand, may be able to!
You'll need some form of photo editing software. Printers often come with some kind enclosed, MS Office typically installs Office Photo Manager or Photo Editor or you can download and install one of these. Those programs will allow you to do basic edits such as enlarge images (except for Picasa!), and some such as GIMP can do a lot more, but be warned that as the size goes up, quality goes down. You can't take a small pic and make it bigger without sacrificing how it looks. There just isn't the information in the small picture to translate well in a larger size.
Best to start with the big picture in the first place. Note that Photobucket will automatically resize images that are just too darn large. After all, even free products come with a price, and Photobucket's is a limit on how big of an image can be uploaded!
Now that I've looked over that list, I think I'm going to download Photoscape and try it out (even though I already have way too many different and similar products).
Gee Joe, you certainly are causing me a lot of work today!