When is a picture “too large” and why?

There is no rule, it all depends on the individual picture, the file format, the quality you are looking for, and other factors. Logos that exceed 20kb or photographs above 100kb should certainly catch your attention, and this means there is probably still some work to do.

The most common issue is using oversized pictures. You should know the largest possible display size and apply it to your pictures accordingly. 

Choosing the wrong file format is another common problem, although it generally affects the quality more than the file size.

Finally, there is a lack of optimization. Image processors such as Photoshop have improved over time in producing optimized images, but it’s usually worth integrating some sort of post-processing into your workflow.