If the later is your problem the solution would include more bandwidth or a faster server or a combination of the both.
Hope that helps...
We don't have pictures but some classes have "PDF" files that students MAY use. My concern is being slow between questions on a quiz or exam. It is one thing to have a couse slow to load but the studetns get fustrated if they have to wait for each qustion to load.
What kind of server are you hosting this on (shared, dedicated, etc)? It may be that many students are taking the same quiz in the same course at the same time, which could slow things down for that quiz, depending on the ability of your server to handle the load.