Hello and thank you for your question.
Text inserted by the transcriber is written in contracted English. The transcriber's note indicators alert the reader to the shift in language so there is no confusion with contractions and foreign alphabet signs. The word "Picture" or even a longer transcriber's note explaining a format to the student will be in contracted English.
One reason for using contracted English in TNs is that the transcriber may not necessarily be fluent in the language, so mistakes that inhibit the students' learning could be introduced.
Another reason is that because the transcriber is in an English-speaking country, it is likely that the primary language of the reader is English and that they are learning Spanish. Even though they are at an advanced level, they may not understand all the vocabulary used in a foreign language transcriber's note and be at a disadvantage in using the book.