Scanning into OpenToonz is a very simple process - but I do not use the built-in tool, which does not work for me.
Instead, download Irfanview first:
http://www.irfanview.com/Make sure your scanner works on your system. My old LideScan 60 has no drivers from Canon which work, and I use the excellent Vuescan instead (which improved the quality of the scans considerably to boot).
It is also possible to use your webcam or other devices, documents, files, and such with a universal Twain driver like UniTwain:
http://www.terminalworks.com/unitwain.
A tad expensive, though. Although if you do need support for webcams or cameras, Unitwain could work for you.
With Irfanview and Vuescan installed, here are the steps:
1) open IrfanView
2) File-->Acquire/Batch Scanning
At this point you must choose to either scan a single image, or a batch of images. A background would be a single image. A group of drawings for an animated character would be a batch.
I will assume we are scanning in a bunch of drawings for an animated character.
3) Click on the "Multiple image (batch mode) radio button.

4) select your twain source first. Click on the button marked "Select TWAIN Source" and select the source. I wish to scan with my flatbed scanner, and I choose Vuescan. If I had wanted to scan frames with my Logitech web camera, I would have chosen Unitwain.
5) The important bit! Set the Output file name to any name EXCLUDING dots, spaces, etc. Then affix a DOT/PERIOD character at the end "."
Next, set the starting counter at 1, the incrememnt at 1, and the number of digits to 4.
It is also possible to remember the last scan counter, which is SUPER handy in case you have worked on a first batch of drawings for your animation, and continue the next day, or you wish to work in smaller batches.
6) Set the destination directory to you OpenToonz project/scans folder. (see screenshot).
Finally, select a OT compatible import format. I choose PNG for lossless images. If you prefer to work with jpg, be certain to check the quality settings under the OPTIONS button before proceeding.
7) Click OK to begin scanning.
8) This depends a bit on the scanner software. In Vuescan I click the preview button, and draw a selection frame for the part I wish to scan. I then click "Scan". I scan the next drawing. Scan button. And the next. Scan button.
If you have a bunch of frames on the same sheet of paper, just preview once, and create a new selection for each drawing, and hit the scan button for each frame.
9) When finished, I close Vuescan. Irfanview will then automatically save all scans as numbered name.000x.png files.
10) Open OpenToonz, and open the file browser. Navigate to your scans folder, and voila: OpenToonz displays your scanned in drawings as a sequence. Drop in a level, and start cleaning up.