Any practical limits would be more a function of the computer hardware than Inquisit. There are a couple of things to consider:
1) Load time
Loading 600 jpgs is going to take a lot of time no matter what software you are using. Each picture must be read from disk, decompressed, and then stored in memory in a format suitable for rapid presentation to the screen. If you repeat this process 600 times, it's going to add up. In theory, Inquisit could shorten the load time by loading the images on demand as the experiment is running, but that simply postpones the delay until the trials are running, and it would throw off the timing of the trials.
2) Freezing
The periodic freezing is likely because the experiment is exceeding the available memory, so the computer is caching memory contents on disk. You can determine the amount of memory your experiment is using by loading the experiment into the Object Browser, and then launching Windows Task Manager and viewing the memory used by the Inquisit.exe process. Memory usage is going to be much higher than 600 x 150kb, because jpgs use a compressed file format, so the uncompressed in-memory image will be larger.
There are a few things you might do to mitigate the freezing:
1) Shut down all other unnecessary programs before running the experiment
2) Reduce the height and width of each image so that it is only as large as the size it is presented on screen
3) Add more memory to the computer
Hope this helps,
Sean