Hi Wayne

Short version

A good starting point would be 1600w x 1000h in JPG. And choose a motive that looks nice when cropped. If smaller it will look grained on large devices. And if larger it will be too slow for some connections.

Long version

It depends on the layout, device and bandwidth.

Image ratios and cropping

For most images (except the post layout and images in markdown) the ratio is not fixed, as they adapt to the device/browser size. Just like you know it from the SPREP website when you resize your browser window.

We do not have fixed image sizes on GBIF.org either. We manage to find images that work when cropped, but I'm certain that it can be difficult at times.

Format

If it isn’t an image, but graphics, then use SVG (instead of JPG) as that scales nicely and is small in terms of bytes. The image that you currently have (the light green background with an arc) looks like a perfect candidate for an SVG.

Resolution

Currently we serve one size for all devices. So, resolution is a compromise between download speed and crispness. Using an image from the SPREP site as an example, we see it is 1920x1500 pixels and 372kb. That would be perfectly suitable for a header image on this site as well

Coming changes

I hope to address resolution and formats in the following days or weeks, in such a way that high resolution images are expected to be uploaded, and they will then be scaled to more appropriate sizes and formats.

We can add more layout options

If your designer has a specific layout in mind (e.g. with fixed ratios) and it has general use, then I'm happy to help implement it or discuss it. 

References