php image save/resize/show practices

[fastman]

What are the best practices on image handling on a php driven web site when several versions of the same image are required (different sizes), bear in mind it's a high load website:[*]Upload and automatically create all sizes needed[*]Upload a single size image then call a single script which based on a parameters resizes/crops and displays image directly to user[*]Upload a single then also with script generate new and cache it for further use (this would require use of file_exists() to check if generated version exists)[*]Call image directly then if it doesn't exist redirect with htaccess to script which would generate it