beagle question

[Amadeus W. M.]

I'm trying out beagle, to see what it can do, and I configured
it to index my home directory, where I have among many things
about 8G of images downloaded from my digital camera. I'm 
doing a search after "img" and most of those images have the
form img_1234.jpg. Beagle does not find a single one of those,
but it does find some img43.gif (from the gimp manual), and 
a few other img??.gif. So I thought maybe it doesn't search the
name, but the contents. I do a 

strings img43.gif | grep img

and nothing.

Thus beagle finds img43.gif because img is in its name (not in
its contents), but it doesn't find millions of img*.jpg. 
What gives?