It all depends, every camera has several formats, and always high or normal quality.. in my camera you can range from 60+ MB for a single pic to around 60 or even less for a small 640x480 in normal quality...
In 2002 for the Geneva Motorshow I had only a small memory card so used the smaller format possible, as above... of course that way on the card you can put lot of pics, but quality is not good..
Now finally I have some better cards and usually I shoot at 1280x960 in high quality or maybe little smaller or if I really need undreads of pics, I use the 640x480 in high quality...
Anyway such pics always need to be resized to obatin a better "quality-to-size" result IMO... especially when you want to upload lot of them... it's better to make them around 60-80 kb each.. maybe not as big as wallpapers so the quality is still good...
I use Phostoshop to resize and compress pics.. basically you can make a 500 kb pic into a 50 kb in 2 seconds
Ah and remember, don't do like I did the first time i took my digital camera at geneva... I had only a set of batteries and after 30 min they were dead (because of the flash mostly)... so especially for big car shows, always bring with you at least 2 or 3 sets of charged batteries