Here are two posters I made for the Ambassadors Wall (Ambassadors are an important institution inside Fedora, people spread around the work promoting Fedora and the wall is a place where they can have their voices heard):
The concepts are similar and straightforward: one is about a cork board where people can put thei message on a post-it note and the other is composed from speech bubbles.
In the end they did not turned in what I imagined initially, I dreamed about having the notes written in a lof of native languages with various scripts and charsets to better show the multicultural aspect and maybe having them written by hand and accompanied by a photo for a human touch (but good look with gathering photos which may be used for print).
Anyway, I am quite happy with the result.
Do your own
Now it you want to customize the poster with your own note/voice, is not easy to add a post-it or bubble, the space is used at maximum already, but grab the SVG sources (cork board, voices), open in Inkscape, delete an existing note, and replace with your. Or do a complete pack of 15 notes.
As some printing house will not be happy with your SVG, here is how to trick them: either export a PNG at the desired resolution, import in GIMP and convert to TIFF or just import the SVG with GIMP and set the size/DPI at import and save as TIFF (for example, an A3 poster at 300 DPI is 3508x4960 px).
Anyone should just shut up when provided with TIFF images.