OK, let's get this straight...
Requests for nonexistent jpg files get redirected. It's a 302, which means either a Redirect in mod_alias or a [R] flag in mod_rewrite. Redirects don't happen by themselves.*
Requests for nonexistent .jhtml get the proper 404. So we don't need to do anything about those.
Have you looked at your logs? When there is a redirect, the original request should be followed immediately by a second request for the name of the file you're being redirected to.
Is it your own server? You can set the error logs to "Debug" level, though it may not help with the 302's.
RewriteRule ^/([a-zA-Z0-9]{2}_([a-zA-Z0-9]{2}))/\2/(.*)(jhtml)(\?)*(.*)$ redirection-url [QSA,NC,L,PT]
RewriteRule ^/([a-zA-Z0-9]{2}_([a-zA-Z0-9]{2}))/webadmin(.*)(\.rss)(\?)*(.*)$ redirection-url [QSA,L,PT]
Is that a direct cut-and-paste from your config file or htaccess? It's an awful mess-- sorry!-- but I noticed one thing. The first rule doesn't actually say anything about a jhtml extension. That is, you say \.rss but you don't say \.jhtml. That means that
anything containing the string "jhtml" would be sent over to
redirection-url [QSA,NC,L,PT]
You don't happen to have files named blahblahjhtmlblahblah.jpg do you?
What does \2 refer to?
* OK, sometimes they do, but not in ways that apply here. For example, mod_speling might kick in if you ask for
fielname.html
and it doesn't exist but you do have a
filename.html
There are also directory-slash redirects and maybe with/without www redirects. But all of those should come through as 301.
