what resolution - JPG's for 500x s.s. ...?

If your images are 500 x 375 (if you are using 4:3 aspect ratio), that is your pixel density. The quality slider when you are saving the jpegs is for the amount of compression, which decreases the file size the lower the number, but also adds noise - like halos around text and such.

Your image size is perfect as far as the resolution. If you are using Photoshop, I love to use the "Save for web..." in the file menu. It opens up a window where you can tweak further. At the top there are tabs where I like to look at the 2-Up view to see the original vs the edited (I am using CS1, so it might be a bit different in your version).

On the left you can select the file type and amount of compression, etc. For jpegs, you can choose the compression amount from the drop down menu below the file type. I always play with it to find a balance between the smallest file size possible while retaining image fidelity. You can see the file size at the bottom of each image's window.

I also like to try .png format just to compare file sizes to the jpegs. If you want to know more about the .png format got to Wikipedia and read all about it.

Once I have found my balance, I hit the Save button and save the file to and "Edited for web" folder I created so I don't lose my originals. I've found that this feature of Photoshop is the one I am using the most lately since I manage 2 websites. I'll even use it to lower the file sizes for keynote presentations or emails - any program where file size is an issue.

Hope this helps. Good luck!

It is all very subjective. In my opinion, anything to do with media whether visual or aural is extremely subjective.

I rarely see anyone use less than 72 pixels per inch if that is what you are wondering. I took a random photo with an original resolution of 2272 x 1704 with 180 pixels per inch resulting in a 1.6 MB photo, and reduced it with photoshop to 500 x 375 at 72 ppi which brings it down to 549K. Then using the save for web dialog on the particular photo I picked I would use the High quality setting because it creates a tiny file of 44K. I chose high because it was the lowest setting before I started really noticing a grainy quality being given to the picture. Some pictures you will be able to get away with more graininess because of the shadows and contrast etc. That is why I play with the quality drop down menu every time. But that is part of the fun for me.

There is no standard anything for editing images for the web. It's finding the best compromise between file size and quality the best way you can. Obviously the smaller the better, resulting in quicker load times. Ultimately it's what looks best to you is all that matters in my opinion. There is no universal absolute minimum file size or resolution. Not in my experience anyway.