YES! We've seen a significant drop in visits via google images when one of our clients eventually changed their file-upload-backend and files were named automatically like 445390348043534.jpg - we then added a separate module only to assure the naming is correct and useful, and are slowly recovering the lost google images visits.

One big site ... what's the advantage of different sites? if you want to make "microsites" or special promotions, you can go with directories, but keep your backlinks together on one single site. Optimize different pages for different terms, keep your internal linking straight, and if you can constantly do something for your linkbuilding, you're all set.

keyword domains are no long-term strategy, but can get great results short-term ... but also check your on-page seo in case there's the real problem ...