I'll start with a disclaimer: this has the potential to cause serious problems in your printer and the fact that it caused none whatsoever in mine should not be taken to imply it'll be safe for yours.
If you're still reading, this is how I managed to print symbols on the Post-It tabs, like this:
GTD devotees could use these for context lists. The rest of us will just enjoy having cute little pictures poking out from between our pages.
You'll need a laser printer, some Post-It tabs (I used the ones that come with the Filofax Sticky Notes insert) and some regular printer paper.
You start by making a document in Word or Publisher with some blank boxes, a little larger all round than the tabs themselves. It should look like this:
Note that this document is in portrait layout, so these are tall boxes, not wide. For the Post-It tabs that come with the Filofax insert, you need boxes that are about 15mm x 50mm. You can download a Word file with these all set up if you prefer.
Print this document out. You should now have a sheet with some empty boxes printed on it. Inside each of these boxes, you're going to stick one of the tabs. This bit is important: the sticky end must be at the end that first goes into the printer feed.
You need now to go back to the document on screen and add your icons, which of course need to be positioned to print at the non-sticky end, like this:
These will be side tabs. If you want top tabs, rotate your icons so they're in the same place but upside-down rather than side-on. Incidentally, all the icons I am using come from The Noun Project, which has a huge range of royalty-free icons. If you prefer, you can download the ones I'm using in this Word file.
Now put the sheet back in the printer, with the blank tabs attached. Make sure the sheet is the right way up for your printer and that it will feed sticky-end first. Print the document that countains icons in the boxes, and when it emerges from the printer, it will have lovely printed tabs attached. You might want to experiment with just one tab to begin with to make sure you are positioning the tabs and icons properly within the boxes.