1) Record a macro with iMacros on Socialoomph of the scheduling process, at least that's what i did. Bulk scheduling is not free there, but you can circumvent that by playing the macro in loop. Scheduling 20k tweets will take some time, but at least it's free this way. I tried it with other services like Tweetdeck, but Socialoomph worked the best.
2) IFTTT can work too, you might want the trigger to be an incoming mail in your Gmail, you can schedule emails with the Boomerang Gmail addon. So practically with scheduling the emails, you can control the timing of your tweets.
You'll need a Gmail acc, install the Boomerang addon, schedule your emails with it. Go to imgur, upload your pics, you can upload 150 at once, after it's done from the source of the page, you can extract the 150 direct urls to the images, these will be needed for the IFTTT recipes. Imgur is just an example, it can be any other image host, where you can get a direct url to the pic. Then head over to IFTTT, link your Gmail and Twitter acc to it and make a Gmail to Twitter recipe, where a new incoming email is the trigger and posting an image tweet is the action. Easier done than said.
Recipe making also can be easily automated with iMacros, 20k tweet is not a small amount, but the macro will work fast, so you can schedule them in no time.
I'm not sure about Dropbox though, IFTTT doesn't support scheduling/timing natively, so you need to be a bit tricky when it comes to this. You need something to trigger the action, with my above example the trigger is the new incoming email in Gmail scheduled with Boomerang, with Dropbox what would the trigger be exactly?
If you want something for free, you need to work for it a bit.
