With regards to sizing images for the forum banners
@Negentropic , generally best practice is to ensure that most of what you want seen in your banner is visible at smaller screen sizes, rather than looking optimal for those of us on desktops and laptops with wide viewports, because 75%+ of folks visiting here are actually on mobiles with pretty low horizontal resolutions.
As an example, if you go to Other Drugs and look at the width of the shortcut buttons I placed in there (below the main banner), they give you a rough indication of the max width you would want to be aiming for for all the main or most important visual content you're planning to show in your image (eg any text with the forum's name on it etc).
You can easily assess how an image will appear on any viewport (screensize) by just adjusting the width of your browser manually - XF will auto-resize the screen down to the smallest mobile width (and which will be almost exactly how it will look on most other modern mobile browsers).
It is possible to get XF to just shrink the image in proportion as the screen resizes, using CSS, but because the vertical dimension ideally needs to remain fixed in all modes, this results in the horizontal distorting quite severely to accommodate the changes.
For example, if you drop into Psychedelic Medicine, you can see how those scrolling images I put up for mr peabody are horizontally adaptive and will stretch to fit their entire contents on any screen. However the distortion you get at higher screen resolutions was an unfortunate but necessary compromise to get the scrolling working smoothly. It looks perfectly acceptable in most mobile resolutions though, so most people won't see an issue. But ideally you'd just want to stick to resizing any images the same way most banners do at the moment.