Getting image sizing correct for your email and webpage headers has always been a fine balancing act between making it as foolproof as possible, without dumbing it down so much that you don't have control over your images.
We knew that some users had found re-sizing their own images to 600px wide before upload to be tricky. But we didn't want to go the usual route of auto-sizing smaller images up to fit (often resulting in a drop in quality).
We've now decided on what we think is a great compromise - Seatrobot now downsizes images larger than 600px wide automatically to fit (making sure that the quality is still good) while maintaining their aspect ratio. Images smaller than 600px wide are NOT changed, but are centered within the email / webpage banners.
In all cases, before presenting on the emails / webpages, the images are padded with a subtle spacing to make them blend with the background.
Hopefully, these changes will aloow you to upload any image you like without having to edit it beforehand - and then look at the result to see if you like it. Then you still have the option of editing your images based on the result (usually due to images being too square) if you want to.
Bottom line - from now on, take any image you like and upload it. You'll see on any rsvp page or test email quickly whether you like it - and if you need to make adjustments, they should be obvious and easy.