Are there any cheat sheets listing all the cars in GTA 4, with images?
There's a useful list on gta4.net. It provides a list of all vehicles with some basic data (top speed, breaking, etc.), and next to each vehicle is a small camera icon which will display an image of that vehicle when you hover over it.
Another useful list is available on ign, select a category to see a page with a list of all the vehicles under that category, their attributes and a screenshot.