One of the features I miss most in Joomla! is the ability to define multiple image sizes and use these in your template, just like you would in WordPress.
One solution I use is a plugin developed by S2 Software: Joomla! Resize Image on the Fly and Cache
With this plugin you can define custom dimensions for the default image fields and define custom sizes for other purposes. It is also possible to use the plugin directly in your template files.
Recently I ran into a problem with this plugin, that was caused by an open_basedir restriction. This is not an uncommon problem for Joomla! sites hosted on a shared server and it can be fixed by hacking a core file. See this tutorial for more information.
One major disadvantage of hacking core files is that every update of the CMS requires you to make the necessary changes again, aside from it being bad practice in general.
When this plugin is the only one affected by the open_basedir restriction, you can fix it by manualy creating the cache folders. The default setting is
images/cache and a subfolder called
remote is also required. Also, make sure the permissions are set correctly, so that the plugin can write to these folders.