What size should the showcase images be for best result?
Upload them as big as you can, and the system should downsize them as need be. Let us know if you encounter an issue or have a suggestion for improvement :)
Just a warning regarding the statement Sigurd made:
If you're uploading large images, the server might run out of memory real quick... this happens frequently with images straight from digital cams and a low PHP memory setting.
Imagine a common 12 megapixel image: 4000 x 3000 pixels â‰ˆ 46 MB Ram. Together with the memory used by SilverStripe, this can lead to memory shortage real quick.
I usually recommend users to upload images in sizes about 1024 x 768 or 1600 x 1200. This is large enough for most purposes but doesn't consume that much memory and upload time. The problem is, that some users don't know how to resize images or that it's a cumbersome thing to do for lots of images. In this cases, I recommend the usage of a batch tool like Automator (Apple), Photoshop Droplet or other.
Edit Ugh. Stupid me. I overlooked the fact that this is about the Showcase images. I guess the SilverStripe Server has plenty of memory available, so go ahead. My post was more of a general warning regarding large images and SilverStripe Websites.