high_resolution_icon_url... But I wasn't able to find any information about any of those images in the documentation. Are any of them fixed size? And/or is there a place where I can find the intent for each of those images?
"Are any of the fixed size?"
I can't really answer that question directly, but for reference, the dimensions of the images supplied by the API for Stack Overflow are as follows:
Image: Dimensions: ===================================== logo_url 250x61 icon_url 158x158 favicon_url 16x16 high_resolution_icon_url 316x316
I didn't have the patience to verify if the other sites used the same dimensions - but I would assume that they do.
"And/or is there a place where I can find the intent for each of those images?"
There is nothing in the documentation (that I am aware of) indicating specific intents for these images. But here is my humble opinion on each of the images:
logo_url- this image contains not only the site logo but also the name of the site in the appropriate font. I can't really see a valid use for this that wouldn't violate the trademark guidelines, however - but you never know.
icon_url- the primary purpose of this image (judging from the filename) is to serve as an
apple-touch-icon- an image that will be used by iOS devices when a page is bookmarked. Of course, there are a lot of other things such an image could be used for.
favicon_url- this image is provided in
.icoformat so I can't really see much use for it.
high_resolution_icon_url- basically the same as
icon_urlbut a much higher resolution. This might be valuable on high resolution displays or an area of your site that requires a really large copy of the logo.