Millions and millions of images are sent via messaging apps daily. To ensure they don't strain servers, most messengers, including WhatsApp, compress the files you send. But Signal is now introducing an option to send 4K images when you don't want to destroy the quality.
The option to send higher-resolution images is currently available in the latest Signal beta v5.11.0 — you can grab it from APK Mirror. Once you have it, you don't need to tinker around with any settings. While sending an image, simply tap the image icon in the bottom left and select High to send 4K images.
If you're trying to quantify the quality difference using this setting, here's an example. An image with a resolution of 6944 x 9248 pixels, weighing 11.6MB, reaches the recipient with a resolution of 3075 x 4096 pixels and a file size of 1.4MB. When sent with the Standard setting, the same image is reduced to a resolution of 1201 x 1600 pixels and a file size of 204KB.
While the difference between these images may not necessarily be perceivable, it's good that Signal offers its users an option to send higher-quality pictures. For what it's worth, Telegram lets you transfer images in full size. Technically, WhatsApp also allows the transfer of higher-res images if you send them as a document but this isn't as straightforward as we'd like it to be.