I posed for a picture at [event X], how come I don't see my picture in the gallery? What is the "NSFW Filter"?
The more risqué pictures are put behind a "Not Safe For Work" (NSFW) filter. The gallery does not display NSFW pictures by default and a "Confirmed Adult User" account membership is required to turn off the filter. At the top of right-hand side gallery navigation (which slips down below the thumbnail block on narrow screens) there is a link "Turn off NSFW filter" - click it. If you have a "Confirmed Adult Account" the page will be reloaded with the NSFW pictures and the link will now say "Turn on NSFW filter". The setting will stay until you close the browser or click the link again. You can set the default for the filter by going to "My Account"-->"Edit" and looking for the "Disable NSFW" checkbox in the "Public photo gallery settings" section of the form.
You still can't find the picture you are looking for it's most likely there was something wrong with it (out of focus, flash didn't go off, etc.) or there were several very similar pictures and only the best of the bunch was used. Also, after three months, photos that haven't been voted above a cut-off threshold are automatically removed (there is more about this in the section on the rating system.)
Simply stepping into the photo area does not in any way guarantee that your picture will be on the website. The only way you can be absolutely assured to get a copy of your picture is to buy a copy (print or digital) in the photo booth.