On a folder by folder basis you can use Sort by File Type in Bridge.
In addition to that you might open a new window, set the preview quality to 'prefer embedded' (low quality but high speed) and select the main folder containing all your folders you want to search for. Then from menu view choose 'show items from subfolders and let it build for a while.
And since images normally have less different file types you might want to search for jpeg, tiff, PSd and PNG first. You can do so in the filter panel under file type.
Another approach in Bridge is using the Filter panel, which I use a lot. Like Omke said, you can have Bridge evaluate all nested folders at once by selecting a disk in Bridge and turning on View > Show Items from Subfolders. In the following screen shot you can see I located hundreds of video files in nested subfolders, and then I selected the video formats in the left column so that I only see the video files in the Content panel and can then work with them (drag them into predefined collections, apply keywords, move them, etc.).
As far as the Finder itself, the search criteria can actually be pretty effective. In the screen shot below, I have set the Kind criterion to Movie and it has picked up two different video file types. Also, it is set to search only the Lab disk, which is not my system volume. If the Kind criterion is missing a file type, you can add more Name criteria that look for specific filename extensions. Then you can save it all as a Saved Search so you can do it again in the future without having to build all the criteria from scratch.