Strange Rendering with img_tag

I need some advice on img_tag()
I checked in the console, and the file name of the image is stored
correctly.

puts a.file_name

dog_whisper_lowres.jpg

The file is stored on the file system itself. In public/images/

However, when the page is rendered, there is a strange string appended
to the image name in the img element, like so:
<img alt="Dog_whisper_lowres" src="/images/dog_whisper_lowres.jpg?
1185178783" />

My rhtml is this:
<%= image_tag("/images/#{@asset.file_name}") %>

Am I missing something in the img_tag function?
I just noticed this today. The images render fine in a web browser,
but that certainly isn't what I want to have in my html!

Any ideas from anyone?

John Joyce