"Am I right in thinking that the weather/ delta T are not in my favor for stud wall analysis during the summer?"
This is a question many new thermographers have. If you think about the heat transfer, the answer is you should be able to get good images from an external wall providing you have sufficient Delta T.
Right now it is hot outside. So, if you are observing the exterior wall of an air conditioned residence, you should have a Delta T of 10°C or 18°F or better, if the interior temperature is about 70°F and the exterior is about 88°F or more. Remember that the air temperature does not necessarily have to be 88°F, if the sun is heating the wall (and has been for a few hours), that will work too. If the building does not have air conditioning and the windows are all open, this would certainly make things more difficult.Here is an infrared image of an exterior wall taken during a hot day (taken from the interior of the building). Notice the hot studs (vs. the cold studs you would see during the heating season).
You can learn more about this application in the Building IR Basics course.
