That's the same animal in my previous post. He ate most of the plant he was on. I dispose of them as soon as I see them.

It looks to me like the yellowing is due to leaf mold (Cladosporium fulvum), a fungal disease; I think I can see spore masses on the underside of one of the leaves, which would clinch it. It's a fungus that needs living tissue to survive, like powdery mildew does, so it won't kill the plant, just make it ratty and miserable. It thrives in high humidity, still air, and warm temperatures.

You have a couple of options... You can spray a fungicide containing chlorothalonil, which will prevent the fungal spores from being able to infect. Chlorothalonil is virtually harmless to people, as it's essentially a barrier rather than a poison... just wash the residue off before eating the tomatoes, and reapply if it rains. This will prevent the fungus from spreading up the plant, but it won't cure the leaves which are already yellow.

You can also (alternatively, or in conjunction) go the sanitation route. Pull off all the leaves or leaflets showing yellow or brown spots, and keep up with it for a few days to make sure you got them all... the plants will look pretty sad, but they'll recover, especially if you feed them. Then take some mulch and layer a couple of inches of it all around the plants, to at least a foot away or until a walkway or bed edge interrupts the soil. Wood chips, grass cuttings, straw, dead leaves -- anything like that will prevent rain or irrigation from splashing soil onto the leaves, which is where the initial infections come from. If you've pulled the infected leaves off the plants, and mulched well under them, you should be able to get it down to a manageable level. Just take off any infected leaves after that, or spray with chlorothalonil.

Prune the plants if necessary or pull leaves out of the middle to make sure there's good air circulation through the plant.

At the end of the season, pull up all the plants and collect any debris such as fallen leaves, and throw it away so that it can't stick around to infect new plants next year. In future years, make sure you've buried or cleaned up all old tomato debris, and mulch well under and around the plants as soon as you get them in the ground. That will help a lot.

The guy in the last picture is a tobacco hornworm, and it's not related to the yellowing. Hornworms just eat, and eat, and eat. If you see missing leaves, or whole stems munched down to stubs, that's the culprit. Pull them off as soon as you see them and squish them, but they won't cause any damage to your plants other than the obvious defoliation.

Hope that helps!

--Alison