I use Jasny Bootstrap for file input: http://www.jasny.net/bootstrap/javascript/#fileinput
Is it possible to specify the input file type to accept only images? Like this:
<input type="file" name="pic" id="pic" accept="image/gif, image/jpeg" />
Yes, this is possible, since it's a standard attribute value for the input tag. Jasny doesn't change anything about that.
However, you should be aware that the accept tag does not work in all browsers: http://caniuse.com/#feat=input-file-accept