redis keys are binary safe. string, JPEG file and empty is allowed.
Even when the task at hand is to match the existence of a large value, hashing it (the long string, for example with SHA1) is a better idea, especially from the perspective of memory and bandwidth.
Try to stick with a schema. For instance "object-type:id" is a good idea, as in "user:1000". Dots or dashes are often used for multi-word fields, as in "comment:1234:reply.to" or "comment:1234:reply-to".
The maximum allowed key size is 512 MB.