That’s a good reason to shoot in RAW - you change the settings and export as a jpg (usually) so your original is always there. I still go back and reprocess shots from a couple of years ago to compensate for all that I’ve learned in that time. On many of my old shots, my composition was great, but the processing was awful.
Canon EF-S/ nomenclature, important for which body it’ll fit on
55-250mm/ focal length, tells us this is a zoom, and a telephoto (longer than normal human eye view)
f/4.0-5.6/ maximum aperture, how big a window you open when you release the shutter f/4 @ 55mm, f/5.6 @ 250mm, as noted, not a very ‘fast’ lens. A constant maximum aperture of f/2.8 would open a bigger window, allowing faster shutter speeds in good light, and better low light shots, but at a price (heavier, bigger, $$$)
IS/ image stabilization, almost essential for wildlife, best turned off on a tripod
Arr, they’re more like guidelines/Barbosavoice
Hey OP, here’s what you can do with a better lens. I shot this on my humble Canon EOS 350D/Rebel XT using a rented 300mm lens (this one, $54/week!) plus a 1.4x teleconverter = 420mm * crop factor = 672mm. It was shot right at sunrise, note the warm light and all the shadow detail in the feathers. (Also note that the image was bigger and much sharper before Photobucket decided to muck with it. Grr.) I still had to push the ISO quite a bit at F5.6, and the XT had a noisy sensor so the details are a bit gritty, but that long focal length was still awesome to have available.
*yeah, we do talk about equipment a lot, huh? 
…to the OP: don’t worry about gear. Not yet. Better glass always helps. But you can still do magic with the kit lens and your Rebel. Better glass won’t help if your exposure is wrong and you missed you missed focus. If you can get focus right with the kit lens: you will never ever miss focus when you upgrade to L-series lenses. A quarter of my portfolio was shot with a Rebel and the kit lens. About half of it was shot in JPEG only! Learn the basics of exposure first and learn what you need to do to nail your focus. Shoot the heck out of your basic set-up until you figure out what the next step lens wise will be. Photography is a personal thing: your next lens purchase should be one made that is best for you. My first lens purchase was the 50mm 1.8 due to countless recommendations: I hated it. 
Different strokes for different folks.
Just keep shooting and coming back to here for critique. Read this newbies guide. Don’t fall into the gear trap…yet. There will be plenty of time for that later on! Just nail the basics, shoot in RAW and JPEG (to give you a comparision image) and use the free RAW converter Digital Photo Professional (free with your camera from Canon) as your editing software for now. Get your exposure right. Nail the focus and eliminate camera shake. Composition will come naturally after a while. And just keep shooting!
To me the shots in the OP can be improved quite a bit in postprocessing. Some selective sharpening (don’t go crazy!) and contrast adjustments can help some of these quite a bit.
Good advice above about shooting at a higher ISO. For birds in flight you need to keep your shutter speed faster.
A good rule of thumb when composing or cropping a wildlife shot is to leave some empty space in the direction that the subject is moving or looking. Your crop of the bobcat picture might be stronger if you place the bobcat at the left side rather than the right. Ideally you would include more of the foreground if the foreground were not so focused and distracting.
Keep at it! And consider a different image host. Flickr is popular and free.
Nice Job for your first attempt. I really like the bobcat image and the birds are pretty good. The one on the beach is nice because it shows habitat as well. If you are interested in more bird photography tips I wrote a comprehensive how to photograph bird post here http://www.photobotos.com/shooting-birds-in-flight-top-ten-killer-tips/