In this other Post there's also information to this topic
Portlets not rendered when running liferay on other context than rootThis one is fixed in liferay 4.2.1
BUT:
I think I encountered another Problem:
When using a different context than root no images are shown in articles based on Structure/Template Basic-Image-Text

Here's what I have done to change context from Root to myPortal. As soon as I can confirm my eMail adress I will also change the Wiki Article Jorge mentioned.
1. stop tomcat and change $CATALINA_HOME/webapps/ROOT to $CATALINA_HOME/webapps/myportal
2. (TOMCAT 5.5) change $CATALINA_HOME/conf/localhost/ROOT.xml to $CATALINA_HOME/conf/localhost/myportal.xml
3. In myportal.xml change/add ...Context ... docBase="/myportal" ...
4. In $CATALINA_HOME/webapps/myportal/index.html redirect to /myportal/c instead of /c
5. In portal-ext.properties add portal.ctx=/myportal
6. modify $CATALINA_HOME/webapps/myportal/WEB_INF/web.xml:
change the root_path from / to /myportal
7. start tomcat
By the way: Uploading the Images again AFTER changing the context did not help.
The source of the rendered HTML Page shows: ...img src="/image/journal/article?...
My System:
Tomcat 5.5.20
Liferay 4.2.1 with classic theme
MySQL 5