In Simpleplanes There have been many cases when letters, words and/or custom logos have been put on aircraft. The problem is that there is no way to make/add a detailed image, word, or sentence on an aircraft without using the parts available, thus increasing the part count, causing more lag. In simplerockets 2 there should be a Simple yet complex way of creating a single image (Or use of image files) and putting it on your rocket. For this I recommend a paint tool (with pre made shapes and the ability to add layers) Think of something like MS paint.
I hope this is useful to you, and I hope you consider it, Thanks for reading.