How to do a compound conditional in httpd.conf?

I have the following in my conf file that sends the no-cache setting if
the alias is not images.xyz.com and this works fine:

SetEnvIf Host "images.xyz.com" ImageAlias
Header append cache-control: "no-cache, must-revalidate"
env=!ImageAlias

Now, I need to add to this so that it is only set for jpg and gif. I
know how to test for an image object:

SetEnvIf Request_URI "\.gif$" IsImage
SetEnvIf Request_URI "\.jpg$" IsImage

.... but how do I do something like (pseudo code):

<if (IsImage is set) then
SetEnvIf Host "images.xyz.com" ImageAlias
Header append cache-control: "no-cache, must-revalidate"
env=!ImageAlias
<end if>

Regards