You can create the equivalent of an AND search when doing Smart Searches. I just tested it by creating a folder that searches for images and only included .jpg and..png files. When clicking the + button in the search window to add search criteria Option clicking the + sign brings up an option that can be set to "Any", "All" or "None" where criteria can be added to create a boolean search.
Try the following as an example:
1. Start a Smart folder and set KIND to Image and all
2. Option click the + sign at the right of the window
3. This is where the any, all, none option appears. Set criteria to "Any"
4. Set the first drop down to file extension .jpg.
5 Click the + sign to add file extension .png
This seems to work. In my case it excluded several .tiff It took a little experimentation to get the Any, All, None settings correct. I wish they would just use the regular Boolean terms.
Advanced searches in the Finder | Macworld