I've designed a grid-based portfolio theme for a photographer; for each portfolio item, I need six different images, all of which have a fixed size. However, no two sizes are alike. Is there a WordPress plugin that will easily allow me to specify a number of different image sizes and choose easily between them when uploading photos? Thanks!
Have you looked at add_image_size() ? It allows you to create new custom thumbnail sizes for uploaded images.
If you need to create new thumbnails for previously uploaded images, use my Regenerate Thumbnails plugin.