Alright! Ready for some CC!?!?
Nice car but I can see room for plenty of improvement to show it off a lot better!
Firstly with the S15 shots, you're in desperate need of decent flash-fill, which is quote literally filling the shot with some light as the front end is in the shadows (Yes i did notice you used flash in the first shot).
The second bunch are kinda dull, i can relate with the bird on having nothing to shoot so shooting the pet (unfortunately we have a cat), but in the second shot it's very soft.. wholllllly crap, take your 50D off auto too.. you green boxer (look at the dial - it's a green box

). The first shot is a bit "wtf a blurry tree-line" second shot is sharp at least, third is very soft, i can see why too - shot at 1/15th, ISO1600.. time for that flash-fill again.. and the last shot of the fish, it's focused on the tank not the fish - that means it's time for manual focus.
Trick for flash-fill for your on-board flash is this - Use as minimally as you can, direct flash almost always looks terrible, pop the flash and use the iso/flashpower button to pick flash power adjustment, roll the aperture wheel (that's what i call it, menu wheel? select wheel?) to dial down or up, look on the top LCD to notice this, then practice. Using auto will usually run higher-than-needed ISO to keep the shutter fast and use 100% (or 1:1) power flash to blast the F--- out of everything to do the same, all in the aim to keep it sharp but makes it look awful.
Grab your manual and look on the index about flash power adjustment as it may have changed from the 40D to the 50D on how to adjust. Remember to adjust it back to 0 though as the next time you use your camera no doubt it'll be a mad rush and you'll forget to change it back and you'll need 1:1 power and it'll still be dialed down.
Also read up about IS on your lens, learn what "f. stops" are (easy as) and then remember how many stops your IS will be good for (it's three but read it anyway), learn what ISO is and why it's not good to let it go too high.
That's this weeks homework

. Now i hope I didn't offend you because if I just wanted to insult you I wouldn't have spent my time writing all that and no I don't think I'm the worlds best photographer, I just thought maybe the small amount of knowledge I have may help you a little rather than no body really posting anything at all.