When developing instruction with both text and images, I think it is actually easier and a lot more efficient than when creating instruction which contains just one medium. Text by itself requires a great deal of explanation and detail to ensure complete understanding. Images by themselves leave a lot to the imagination and the potential for misunderstanding. Either by themselves may also exclude a particular learning preference, which leaves learners struggling with an instruction not geared to their strong suits. Together the two ensure that instruction is complete and detailed without being overly wordy. It also produces an instruction which can be effectively used by most learners.
I use the dual instruction of text plus images quite often. Although my strong suit is text instruction, it is natural for me to include illustrations or screen shots which better explain what I am trying to convey textually. First of all, many of my learners prefer to learn through visual modes of instruction, and the visual reinforcement helps them picture what I am trying to get across. Secondly pictures save words. You can say with a single picture what it would take many words to explain. Third, the combination of the words and the picture reinforce the concept and ensure clarity.
Sometimes there are limitations to using pictures. For example, I often send instructions to people via email, and they may be receiving that email on a mobile device. I have often been told by a learner that they did not receive the pictures because they were looking at my email on their phone. When creating an instruction which is going to include both text and images it is important to know how that instruction will be delivered. Pictures add bandwidth and size to a document which may limit how it can be delivered to someone who has a limited internet connection or is receiving the instruction over a mobile device.
There are, however, many benefits to the dual modality, including increased clarity, fewer words, better design, increased engagement, and reaching different learning preferences to name a few. This is one of my preferred methods of instruction for this very reason.