How to find all images in folder
I get frustrated using the Finder application to find anything. I guess I don't have a mind that works like the authors of the Finder application. If you are used to using commands you can try the following. 


Open a Terminal application window.
Use the cd command to change to the base folder where the files
are. The command pwd will give your current folder.
Enter to the command

find . -name "filesToFind" -print


and the current base folder and all subfolders will be searched.


For example to find all files ending the .txt on your desktop, open a Terminal window and type:

cd desktop
find . -name "*.txt" -print


Note: The instructions for the find command can be found by typing the command man find or at this link.