Using text elements in your file isn’t totally out of the question when uploading illustrations; the definitive line lies in the subject. If textual elements are used as the primary subject to sell an illustration your file will be declined as a derivative work.
Keep in mind, many fonts and dingbats are protected by copyright, so please ensure you research carefully to ensure you are not stepping on anyone's toes when you use existing fonts in your image. Illustrations containing the distinctive and recognizable characteristics of copyrighted fonts will be declined for copyright reasons.
If you are using a font as a minor element within your illustration, it's imperative that you convert the font to paths before submission. Not everyone is guaranteed to have that exact font file in their font library. If you don't convert your font it could default to a system font when opened on another computer. Submissions containing unconverted fonts will be declined.
Example of unconverted text defaulting to a system font:
