There are a lot of websites listing info and pictures of tomato diseases, unfortunately the photography for many of them is often not very helpful.
This is the site I have found most useful
It often does not come up on searches and I had to search my hard drive for my saved copy to find the web address.
I saved the file to my computer and downloaded all of the larger linked pictures, and then in my word processer (Word) I opened the file and pasted the large images as large as they would fit and saved as a single file web archive (.mht) or .doc. Now I just open my file, look at the little pictures to get some idea of the problem and then scroll down my page to the big picture to confirm. I also add informative pictures from other sites.
Sites for mineral deficiencies
Hope this helps,