Two images posted side by side can appear slightly different even if they originated from the same photo file. This usually comes down to how the image is handled behind the scenes. When an image is uploaded directly to the forum as an attachment, the forum software processes it to fit within size and performance limits, which can include recompression, thumbnail creation, and removal of some embedded data such as color profiles.
When an image is linked from an external site such as Flickr, the forum does not process or modify it at all; the image is delivered directly by the external host using that service’s own image handling, with the browser simply scaling the image to fit the page if needed.
Because attached images and linked images pass through different systems, small visual differences can occur, and this behavior is normal and applies equally to all users. For best results when uploading an attachment, exporting the image locally at or below the forum’s maximum size (1600 pixels on the long edge), using the sRGB color space, and moderate JPEG quality helps minimize additional processing by the forum’s image library.
For images linked from Flickr, selecting a “Large” size (such as 1600 pixels on the long edge) provides a good balance of quality and page layout, while allowing Flickr and the browser to handle scaling cleanly without forum-side processing.
Xenforo 2.3 offers image optimization via WebP although there are trade-offs (older browsers don't support WebP, conversion to WebP can result in double compression, i.e, original jpeg compression followed by WebP compression.) We currently are not using this option.
without resizing
