Grazie, in your case it's much more likely to be how your iPhone formats the pictures and what extra meta/header information it stuffs in the files. All that extra garbage could be what's hanging up Vegas.
Also keep in mind that the file size of the images really doesn't mean much of anything at all. Vegas must decompress them in order to use them, so in that regard a 1MB 2400x3600 image is really the same size as a 6MB 2400x3600 image.
I've done Vegas projects with over 11,000 1152x2048 JPG images without a hitch. In my case i know the images contained nothing other than the minimal JFEF header information which has image dimentions, resolution, and compression factors in it, no other metadata at all.
I just finished up a 1hrs 20 min blu ray project and I got some Jeg that is 3456x5184 @9mb each, no problem. I must admit I am one the lucky few never got a crash yet on V12 or 11. My PC is nothing fancy, just an old Dell old i7 920 with 12gb of ram and a cheap ass GTX260 card. Although I am not using GPU function. I am not using compatiability mode.
I agree with the post that is your source that is the problem - not the number and size of jpgs on timeline.
There are several things that Vegas seems to get along well with and a .png file is probably the best. Even photoshop format works.
There are variants for all file types and unfortunately .jpg has a very wide latitude of source code from old to new variants.
Personally, I drag and drop very very few still images to SCS VegasPro. Rather all of my slide composition work is done in an excellent keyframe compositor such as Photodex Proshow.