What do you call objects, or kinds of objects, that cannot be described without visual aids?
There is a phrase "beyond words", but it is not usually used for objects, more for emotions or situations.
Oops, logic has just given me a good kick - if there was a word for such objects, then such objects would cease to fall into this category. ;-)
Of course, there are words like "thing", "nondescript", "thingamajig", "visual", "spectacle", "sensory" and "picturesque", and phrases like "to be seen".
A pretty word for things that cannot be expressed in words is ineffable, but if it's describable with a picture, it doesn't feel ineffable. Unless you're talking about qualia. An example would be helpful.
Anything related to the senses, that the person you are describing to has not experienced.
A good example would be explaining colour to someone who is blind or trying to describe a sunset.
Being able to explain physical feelings such as touch, to someone who has never been able to physically feel objects, imagine trying to explain something being smooth or abrasive. If the person has no frame of reference then words are just words.
Highly technical descriptions of motion or construction are theoretically possible with words but it will end up feeling like you are using the wrong tool for the job. For example: Blue-prints for a warehouse or instructions on how to put together a truck engine both benefit greatly from visual aids.
With regards to the logical trick mentioned by Lee, you can easily describe an idea of an object without referring directly to the object itself. The concept of an indescribable thing is alive and well and we can freely talk about it without describing it.