First, let's have an understanding of static and dynamic here:
In general, dynamic means energetic, capable of action and/or change, or forceful, while static means stationary or fixed.
Thus, a picture may have action to it like athletes running on a field could be a rather dynamic picture, like a player catching a ball for example. Another example would be Times Square on New Year's Eve just before midnight where there are a million people out in the street to celebrate the new year.
In contrast, a picture of an empty field without any players may be seen as more static since nothing is happening there.
The terms may be quite correct in terms of offering why "alternative assessment techniques" could be preferred for "linguistic development."
For something a bit more specific, let's consider this site for example. The questions and answers could be seen as dynamic because they can be edited. After all, this section didn't appear when this answer was first written, right? In contrast the name of "English Language Learners" is static as that is how this site is known, the "ELL" stack for a short form. If you couldn't edit the questions or answers then they would be static as they would be fixed and unable to be edited. In contrast, since we can edit these they are dynamic because what you read today may be changed by tomorrow. Does that make it seem a bit more real now?
If you want a more personal example, consider the changes in your body as you grew up. The was dynamic because it changed in size so much. In contrast, if you buy a car then it can be pretty static since it isn't likely to add or remove weight unless you make changes to it.
This is how I'd interpret each word when used to convey an opinion:
Static picture likely is meant to convey old-fashioned, stuck in the past, only one point in time. Think of a photograph shot with a camera where hours could be spent setting up the shot in the old days where a portrait or photograph is taken. Imagine seeing the instructions for how to do a dance.
Dynamic picture likely is meant to convey modern, lots of data, action, energy, many points in time. Think of a video shot done with a smartphone that required little set up and could use audio and video to convey a message. Imagine seeing a short film of the instructions for how to do a dance. Same material but with a much different packaging to it.