Mixbook’s process for making online slideshows is very accessible to English Language Learners. You can grab images off the Net and easily write in text. Plus, you have the ability to collaborate with others on developing the slideshow, though not in “real time.”
Though its interface might be a little simpler than the slideshow creation website One True Media, I’d still stick with One True Media as my favorite. One advantage that site has over Mixbook is that with One True Media you can easily add music and video to your slideshow, too.
Please do let me know, though, if I’m missing something in my comparison of the two.